Updated Xamarin assets
Cleaned up Xamarin project files
Updated icons and layout for Window Phone 8 in Xamarin
Added placeholder icon for Android on Xamarin
Updated Xamarin for iOS 7 look and feel

Refs #232
Merge pull request #275 from matthewdunsdon/update-xamarin-for-ios-7-and-wp8

Updated xamarin for ios 7 and Windows Phone 8