API resources to get, add and remove river followers

* Implemented the service methods to allow followers
to be fetched, added and removed via the API.

* Renamed the river_subscriptions table to
river_followers

* Override hashCode and equals in the Account model. This
is necessary in order to be able to perform remove() operations
on a list with com.ushahidi.swiftriver.core.model.Account objects
Fix river collaborator API resources

* The service methods were using the id of the collaborator
entry in river_collaborators instead of the account id of the
collaborator.