Implemented AccessToken.revoke_for to allow individual token revocation

This would be the first step in resolving issue #303. Feedback on how/if an endpoint should be created? Perhaps something like the originally suggested POST /oauth/revoke with {id: 'test', token: 'tdet3325xe5vD65S'}?

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Alternatively, if a controller endpoint is being implemented. This may be unnecessary as one could just use "doorkeeper_token.revoke" relying on Doorkeeper::Models::Revocable and Doorkeeper::Helpers:Filter::ClassMethods. If so, the question still stands, where to put the endpoint? "Delete /token" perhaps?