Added LaunchUriAsync method to IDevice

Mainly to overcome a bug on Xamarin.Forms WP8 platform in opening non
http(s) scheme Uri’s.

http://forums.xamarin.com/discussion/23273/device-openuri-and-wp8?new=1

The problem can be verified by changing Geolocation view models
GetDrivingDirections Labs IDevice call to Device.OpenUri method. It
will crash the application on WP8 due to invalid Uri.