[WIP] Use Microsoft.Windows.Shell instead of hand-coded solution for WinForms interop

This is the work that @bitterskittles started which should resolve a number of issues spotted recently:

- #496 - HwndHost in MetroWindow
- #488 - WebBrowser not showing up in MetroWindow
- #481 - WinFormsHost not showing up in a MetroWindow 

Other changes:

 - use NuGet package instead of binary (thanks @flagbug)
 - update nuspec - only reference `Microsoft.Windows.Shell` for `net40`
 - by default `AllowTransparency = !EnableDWMDropShadow` - the demo shows how you disable this but it seems like the this should be the default

TODO:

 - [x] include a sample to ensure we can catch regressions
 - [x] making `AllowTransparency = true` by default - [this line](https://github.com/MahApps/MahApps.Metro/pull/503/files#L0R92) 
 - [x] stop using EnableDWMDropShadow
 - [x] feedback from others who aren't running Win8 that dropshadows are fine for the Panorama demo (I think they changed something DWM-related and the dropshadows are no longer used)
 - [x] introduce `AllowTransparency` dependency property into BorderlessWindowBehaviour and replace the related EnableDWMDropShadow usages
 - [x] update demo to use `AllowTransparency` instead

Thanks again to @bitterskittles for getting to the bottom of this.