Added the UserStoreInterface.
Merge branch 'feature_user_store_interface' into feature_em

Conflicts:
	src/AdamWathan/EloquentOAuth/OAuthManager.php
	src/AdamWathan/EloquentOAuth/UserStore.php
	src/AdamWathan/EloquentOAuth/UserStoreInterface.php
Removed configuration option for the UserStore implementing class, after discussing with Adam that the best strategy would be to extend the Service provider and inyect the custom UserStore there.