Example of hosted app using fxpy (bug 1060608)

This is an initial working hosted app that can be used to test web (desktop) payments. It does not address the CORS problems in the packaged app which is a [separate bug](https://bugzilla.mozilla.org/show_bug.cgi?id=1104371)

r? @muffinresearch 

I manually tested both the packaged and hosted example apps after this patch.