Added a RelationshipChecker service that contains a function which only returns true if there is a relationship containing two users
Removed JSON serialisable from Users models
Changed current_user to just an ID number returned by UsersController->meAction
Removed JSONSerializable because jms-serializer makes it redundant
Moved checking of existing relationship to external service

Removed redundant index JSON action

Improved response flow of createAction
Improved error reporting of createAction

Added checking that a relationship exists between source and target
users
Merge branch 'develop' of github.com:daniel-bell/hush into develop