Change "Facade" to "Surrogate"

This backwards-compatible change removes the word "Facade" in class names and comments in favor of the word "Surrogate." All current tests pass.

The word "Facade" implies the [facade pattern](http://www.dofactory.com/Patterns/PatternFacade.aspx), but the implementation is closer to a [proxy pattern](http://www.dofactory.com/Patterns/PatternProxy.aspx).  The word "Surrogate", on the other hand, implies something closer to the existing behavior (i.e., a static proxy call to a container instance).

The method names (get|set)Facade(Accessor|Application) remain in place. Changing them requires changing the related abstract class (_Surrogate_, née _Facade_). Because userland code may be extending the abstract class, that would require the userland code to be changed as well. That is not a backwards-compatible approach. Thus, the method names remain as a historical artifact. Future versions should deprecate and/or rename these methods to remove the word Facade.

Per <https://github.com/laravel/framework#contributing>: Since this is a rename/replace of an existing behavior, and not a new component, this pull request is against the latest stable release. Please provide specific instructions if another approach is preferable.