Extensions moved to Facebook.Client

Adhering to preexisting code, removing explicit
Facebook.Client.Extensions namespace.
Moving our extensions classes to its own directory.

Adhering to preexisting code, we are using an explicit directory for
extension classes.
Renaming existing and generic Extensions file to more specific StringExtensions

In the preexisting code, they use one file per extension class.
Adhering to that.
proj file for iOS updated to new extensions location.
Temporarily adding dependence on Facebook Debug assembly.

This to avoid erors with XS not finding certain classes and so on.
Use EventArgs, not RoutedEventArgs

For compatibility, using the more generic EventArgs. The handler
doesn't use the event data anyways.
Also, using the Facebook namespace (for FacebookClient)
Added Event Extensions class.

For EventHandler.RaiseEvent method.