Support for the channelUrl parameter of FB.init().

Reference : https://developers.facebook.com/docs/reference/javascript/
Migrated the fos_facebook.channel.url parameter to a route, removed an unused parameter.
Changed the RouterInterface to a UrlGeneratorInterface. Formatting changes.
Changed implementation to interface in test.
Changed implementation to interface (bis).
Merge pull request #114 from adurieux/feature-channel

Support for the channelUrl parameter of FB.init().