Add GUIApplication class

This PR adds an Application subclass that uses PyFace to start a UI event loop in a cross-platform way.

The intent is to provide a cheap and simple way to take a TraitsUI (or PyFace, or Qt, or even potentially Enaml) UI and start lifting it to Envisage without worrying about also having to shift it to Tasks.  This allows the developer to focus on getting the model parts of the application properly wrapped with Envisage before worrying about UI.

Includes docs and a simple example.