Rename organization_groups_get to groups_get
Add shortcut for /organizations/{org_id}/groups

Adds an OrgGroupsResource that provides a simple shortcut router
for getting and inserting groups in a specific root organization.

Closes Issue #24 (Plumb /groups resource into the REST API)
Merge pull request #35 from jaypipes/groups

Add shortcut for /organizations/{org_id}/groups