REPO-1307: Create group membership

- Added create group membership functionality
   - Added tests

git-svn-id: https://svn.alfresco.com/repos/alfresco-enterprise/alfresco/BRANCHES/DEV/5.2.N/root@134750 c4b6b30b-aa2e-2d43-bbcb-ca4b014f7261

source/java/org/alfresco/rest/api/Groups.java

@@ -131,4 +131,12 @@ public interface Groups
      * @return a paged list of {@code org.alfresco.rest.api.model.GroupMember} objects
      */
     CollectionWithPagingInfo<GroupMember> getGroupMembers(String groupId, Parameters parameters);
+
+    /**
+     * Create a group member.
+     *
+     * @param groupId the identifier of a group.
+     * @return a {@code org.alfresco.rest.api.model.GroupMember} object
+     */
+    GroupMember createGroupMember(String groupId, GroupMember groupMember);
 }