Add FolderExists and DeleteFolder to IMvxSimpleFileStoreService.

- Not implemented on WinRT
- WinPhone doesn't support recursive delete
(I haven't tested WinRT/WinPhone build or run)

Also, MvxBaseFileStoreService has this comment at the top:
```
#warning THIS FILE NOW COPIED NOT SHARED - WOULD BE NICE TO HAVE A 'FULL MONO' PCL PROFILE
```

Is there a reason that file isn't just linked between the few projects (wpf, touch, droid) that use it?