[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`

TODO:

 - [ ] include a sample to ensure we can catch regressions
 - [ ] 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)
 - [ ] making `AllowTransparency = true` by default (see [this line](https://github.com/MahApps/MahApps.Metro/pull/new/make-windowchrome-not-suck#L0R92) - is that a reasonable default?

Thanks again to @bitterskittles for getting to the bottom of this.