Initial commit (#332).

src/main/java/org/gitlab4j/api/systemhooks/MergeRequestSystemHookEvent.java

+package org.gitlab4j.api.systemhooks;
+
+import org.gitlab4j.api.webhook.MergeRequestEvent;
+
+import com.fasterxml.jackson.annotation.JsonIgnore;
+import com.fasterxml.jackson.annotation.JsonSubTypes;
+import com.fasterxml.jackson.annotation.JsonTypeInfo;
+
+@JsonTypeInfo(use=JsonTypeInfo.Id.NAME, property="object_kind")
+@JsonSubTypes({
+    @JsonSubTypes.Type(value = MergeRequestSystemHookEvent.class, name = MergeRequestSystemHookEvent.OBJECT_KIND),
+})
+public class MergeRequestSystemHookEvent extends MergeRequestEvent implements SystemHookEvent {
+
+    public static final String X_GITLAB_EVENT = "System Hook";
+
+    @JsonIgnore
+    @Override
+    public String getEventName() {
+        return (OBJECT_KIND);
+    }
+}

src/test/resources/org/gitlab4j/api/merge-request-system-hook-event.json

+{
+  "object_kind": "merge_request",
+  "user": {
+    "name": "Administrator",
+    "username": "root",
+    "avatar_url": "http://www.gravatar.com/avatar/e64c7d89f26bd1972efa854d13d7dd61?s=80&d=identicon"
+  },
+  "project": {
+    "name": "Example",
+    "description": "",
+    "web_url": "http://example.com/jsmith/example",
+    "git_ssh_url": "git@example.com:jsmith/example.git",
+    "git_http_url": "http://example.com/jsmith/example.git",
+    "namespace": "Jsmith",
+    "visibility_level": 0,
+    "path_with_namespace": "jsmith/example",
+    "default_branch": "master",
+    "ci_config_path": "",
+    "homepage": "http://example.com/jsmith/example",
+    "url": "git@example.com:jsmith/example.git",
+    "ssh_url": "git@example.com:jsmith/example.git",
+    "http_url": "http://example.com/jsmith/example.git"
+  },
+  "object_attributes": {
+    "id": 90,
+    "target_branch": "master",
+    "source_branch": "ms-viewport",
+    "source_project_id": 14,
+    "author_id": 51,
+    "assignee_id": 6,
+    "title": "MS-Viewport",
+    "created_at": "2017-09-20T08:31:45.944Z",
+    "updated_at": "2017-09-28T12:23:42.365Z",
+    "state": "opened",
+    "merge_status": "unchecked",
+    "target_project_id": 14,
+    "iid": 1,
+    "description": "",
+    "updated_by_id": 1,
+    "merge_params": {
+      "force_remove_source_branch": "0"
+    },
+    "merge_when_pipeline_succeeds": false,
+    "lock_version": 5,
+    "time_estimate": 0,
+    "last_edited_at": "2017-09-27T12:43:37.558Z",
+    "last_edited_by_id": 1,
+    "head_pipeline_id": 61,
+    "ref_fetched": true,
+    "source": {
+      "name": "Awesome Project",
+      "description": "",
+      "web_url": "http://example.com/awesome_space/awesome_project",
+      "git_ssh_url": "git@example.com:awesome_space/awesome_project.git",
+      "git_http_url": "http://example.com/awesome_space/awesome_project.git",
+      "namespace": "root",
+      "visibility_level": 0,
+      "path_with_namespace": "awesome_space/awesome_project",
+      "default_branch": "master",
+      "ci_config_path": "",
+      "homepage": "http://example.com/awesome_space/awesome_project",
+      "url": "http://example.com/awesome_space/awesome_project.git",
+      "ssh_url": "git@example.com:awesome_space/awesome_project.git",
+      "http_url": "http://example.com/awesome_space/awesome_project.git"
+    },
+    "target": {
+      "name": "Awesome Project",
+      "description": "Aut reprehenderit ut est.",
+      "web_url": "http://example.com/awesome_space/awesome_project",
+      "git_ssh_url": "git@example.com:awesome_space/awesome_project.git",
+      "git_http_url": "http://example.com/awesome_space/awesome_project.git",
+      "namespace": "Awesome Space",
+      "visibility_level": 0,
+      "path_with_namespace": "awesome_space/awesome_project",
+      "default_branch": "master",
+      "ci_config_path": "",
+      "homepage": "http://example.com/awesome_space/awesome_project",
+      "url": "http://example.com/awesome_space/awesome_project.git",
+      "ssh_url": "git@example.com:awesome_space/awesome_project.git",
+      "http_url": "http://example.com/awesome_space/awesome_project.git"
+    },
+    "last_commit": {
+      "id": "ba3e0d8ff79c80d5b0bbb4f3e2e343e0aaa662b7",
+      "message": "fixed readme",
+      "timestamp": "2017-09-26T16:12:57Z",
+      "url": "http://example.com/awesome_space/awesome_project/commits/da1560886d4f094c3e6c9ef40349f7d38b5d27d7",
+      "author": {
+        "name": "GitLab dev user",
+        "email": "gitlabdev@dv6700.(none)"
+      }
+    },
+    "work_in_progress": false,
+    "total_time_spent": 0
+  },
+  "labels": [],
+  "repository": {
+    "name": "git-gpg-test",
+    "url": "git@example.com:awesome_space/awesome_project.git",
+    "description": "",
+    "homepage": "http://example.com/awesome_space/awesome_project"
+  }
+}