Add "created_at" in package (#1017)

Fixes #1009

src/main/java/org/gitlab4j/api/models/Package.java

 package org.gitlab4j.api.models;
 
+import java.util.Date;
+
 import org.gitlab4j.api.utils.JacksonJson;
 
 public class Package {
@@ -8,6 +10,7 @@ public class Package {
     private String name;
     private String version;
     private PackageType packageType;
+    private Date createdAt;
 
     public Long getId() {
         return id;
@@ -41,6 +44,14 @@ public class Package {
         this.packageType = packageType;
     }
 
+    public Date getCreatedAt() {
+        return createdAt;
+    }
+
+    public void setCreatedAt(Date createdAt) {
+        this.createdAt = createdAt;
+    }
+
     @Override
     public String toString() {
         return (JacksonJson.toJsonString(this));

src/test/resources/org/gitlab4j/api/packages.json

@@ -3,12 +3,14 @@
     "id": 1,
     "name": "com/mycompany/my-app",
     "version": "1.0-SNAPSHOT",
-    "package_type": "maven"
+    "package_type": "maven",
+    "created_at": "2012-10-22T14:13:35Z"
   },
   {
     "id": 2,
     "name": "@foo/bar",
     "version": "1.0.3",
-    "package_type": "npm"
+    "package_type": "npm",
+    "created_at": "2012-10-22T14:13:35Z"
   }
 ]
\ No newline at end of file