Token revocation implementation [RFC 7009]

## Implementation of the OAuth 2.0 Token Revocation [RFC 7009]

Hello,
Thank you for this excellent gem :+1: 
We use the revocation mechanism internally and we would like to share it with the Doorkeeper community.
It is compliant with the RFC 7009 : http://tools.ietf.org/html/rfc7009 

The client can request the revocation of a particular token by making an HTTP POST request to the token revocation endpoint URL :
```POST /oauth/revoke```

Any code review, comments or questions are welcome :-)