Add mutes endpoints (1.1/mutes/users/...)

Didn't bump version numbers/update package, just added the block for mutes endpoints

If reviewed/accepted, a version bump and push to NPM would be much appreciated!
Based on the endpoint structure Twitter chose, I suspect there may be future paths like "mutes/hashtags" etc. Under that assumption it probably makes sense for the node-twitter-api mutes(..) function to take a type like 'users/create' or 'users/destroy', so that future additions to the Twitter API don't require breaking changes to node-twitter-api.