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

**Description :** 

The Facebook developer's website highly recommends to create a channel.html file in order to address some Cross-Domain issues.

**Todo:**

- `Resources/config/routing.xml` : The route pattern should follow the `fos_facebook.channel.url` parameter of the `config/facebook.xml` file.
- `Resources/views/initialize.html.twig` l.10 : there might be a bug here : do we have to escape the URL ?
- `Templating/Helper/FacebookHelper.php` l.71 : not quite sure that this is the right place to do that...
- Code review welcome for these points...

**Bug fix:** no
**Feature addition:** yes
**Backwards compatibility break:** no
**Bundle tests pass:** yes 
**Fixes the following tickets:** #113
**References the following tickets:** -
**External references :** https://developers.facebook.com/docs/reference/javascript/