Add endpoints for retrieving service contracts

This is a follow-up for THREESCALE-258 (Ability to unsubscribe developers from a service). See this comment.

The ability to unsubscribe from service has been added in the UI and via the Service Subscription Delete endpoint.

However, at the moment it's useless, 'cause it requires a "service contract ID" (/admin/api/accounts/{account_id}/service_contracts/{id}.xml), but I can't find any endpoints that would return this ID.
I suggest we add more endpoints for service subscriptions, so the user can at least list the service contracts and get their IDs.