Initial commit (#325, #331).

src/main/java/org/gitlab4j/api/webhook/ChangeContainer.java

+package org.gitlab4j.api.webhook;
+
+import org.gitlab4j.api.utils.JacksonJson;
+
+public class ChangeContainer<T> {
+    
+    private T previous;
+    private T current;
+
+    public T getPrevious() {
+        return previous;
+    }
+
+    public void setPrevious(T previous) {
+        this.previous = previous;
+    }
+
+    public T getCurrent() {
+        return current;
+    }
+
+    public void setCurrent(T current) {
+        this.current = current;
+    }
+
+    @Override
+    public String toString() {
+        return (JacksonJson.toJsonString(this));
+    }
+}