Reduce code duplication

ping @mac_nibblet @Ocramius 

This PR aims to reduce code duplication for generating the token response (I'm not 100% happy with the naming of "createTokenResponse" and "prepareTokenResponse" though).

You can also see that I add the access token as a metadata of the response. This will be used by the ZF2 module to enable event manager.

If this can be reviewed asap I'd be happy =).