Complete user group membership functionality

Completes Issue #32

Add DB API calls to remove a user from group membership

Adds tests for the API resource layer for:

 GET /users/{user_id}/groups
 GET /groups/{group_id}/users
 DELETE /users/{user_id}/groups/{group_id}
 PUT /users/{user_id_/groups/{group_id}
Merge pull request #39 from jaypipes/groups

Complete user group membership functionality