Provides support for the PATCH method for HTTP

This pull request allows Silex routes to be specified to handle the `PATCH` method for HTTP. Without this, when trying to specify that a route accepts `PATCH` requests, Silex throws a `BadMethodCallException` with the message `Method "Silex\Route::patch" does not exist.`

#### References:

* [PATCH Method for HTTP](http://tools.ietf.org/html/rfc5789)