Basic plugin architecture introduced.

-widget module moved to the plugins folder
-removed Durandal.Router nuget package in favor of including all
official plugins as part of the Durandal package
-fixed up the samples to demonstrate loading and configuring the widget
plugin
Reversed the order of documentAttached callback exec.

Now the documentAttached callbacks will be executed from the inside out
so that child view/viewmodels can do their work before the parent(s).
Implemented errors for system.acquire.

-Added viewEngine.createFallbackView. Now, if a view is not found, we
generate an error view here. This can be overriden to allow developers
to create custom view 404 fallback generation.
-Cleaned up system.error usage.