Async File plugin

With these changes the file plugin has now some async methods. The PCL profile that we use is "async enabled" so async just works immediately.

The async methods are implemented in the base class MvxFileStore. This class is common to all platforms.
There is a new interface for the async methods IMvxFileStoreAsync. Currently this interface is only registered in the iOS project and it uses the same instance than the IMvxFileStore under the hood.

Since I don't Xam.Android and no WP environment I can't make the Plugin.cs change in the other projects (Xam.Android and WP).