Add FolderExists and DeleteFolder to IMvxSimpleFileStoreService.

- Not implemented on WinRT
- WinPhone doesn't support recursive delete
Rename DeleteDirectory implementation to DeleteFolder like they should be.
Fix for cached image file issue - needs further dev testing - e.g. can we load resource files
Add Sqlite plugin for Windows Phone - relies on binaries from sqlite
Merge remote-tracking branch 'mvvmorigin/vnext' into vNextDialog
Simple changes
Merge pull request #104 from danzel/vnext

Add FolderExists and DeleteFolder to IMvxSimpleFileStoreService.
Merge pull request #1 from slodge/vnext

Vnext