Issue 156. moved Membership management to a new class MembershipManager.
added methods to create Membership entries for users and groups.

added group_id to the JSON body if group is present in Membership instance.
added @Ignore to the group membership test. it's a bug in Redmine 2.5.2
Merge pull request #157 from taskadapter/ask-156-group-membership

Issue 156. moved Membership management to a new class MembershipManager.