BusyIndicator on SimpleProgressPage working. (Closes #7)

-Left in some of the architecture for the generic content busy indicator
 on the shell view model. But this code is inherently buggy, as it really
 requires the Caliburn Micro bindings to work properly (and they don't).
 They probably do, but I am not using it right. Anyway, I can't bind to
 the Main_IsBusy property in xaml. The workaround was to have a
 ContentIsBusy property exposed in the shell view model. But when you
 nav to another page, the is busy stays up. Anyway, I think I know how
 to get it working, but for now, I just implemented the IsBusy directly
 on the SimpleProgressPage.

Signed-off-by: William Raiford <bill.mybiz@gmail.com>