Added group_membership_id to Groups API

Also added condition to retrieve only 'available' groups and 'accepted' group_memberships
Changed workflow_state condition to != deleted

There could be other states other than 'accepted'
Simplify Group API: query GroupMembership directly
Merge pull request #1 from va7map/group-api

Simplify Group API: query GroupMembership directly
Merge pull request #120 from ronvs/group-api

Added group_membership_id to Groups API