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).
Added composer.json
Merge pull request #152 from Spea/2.0

Added composer.json for 2.0 branch
Switch variables from private to protected
Merge pull request #161 from tiois/2.0

Switch variables from private to protected
[Doc] Updated FacebookSDK repo

The previous version of SDK has been deprecated
Merge pull request #173 from franmomu/patch-1

[Doc] Updated FacebookSDK repo
Merge pull request #114 from adurieux/feature-channel

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