Implementation of the RFC 7009 - Token Revocation

Update revoke action acording to the RFC009

Move same credential? method in the access token model
Add revoke route
Add revocation token endpoint url helper
Test the same_credential? method
Test the token revocation flow
Restrict the token params to be only passed in the POST request body.
Does not use the token_type_hint logic anymore
Refactoring of #same_credential? and tests
Rewrite #revoke_token to be compatible with Mongoid
Refactor revoke token flow specs
Refactor and complete the #same_credential test
Always respond HTTP Status 200
Token revocation implementation [RFC 7009]
Refactor and complete the #same_credential test (2)
Remove some code offenses
Merge pull request #374 from ifeelgoods/allow_to_revoke_a_token

Token revocation implementation [RFC 7009]