Updated iOS deployment target to iOS 5.1
WinRT: added project files for Windows 8.1 and Windows Phone 8.1 targets
Made Win[Phone] 8.1 project file names more consistent with 8.0 counterparts

This renames the two, main pairs of SDL_image project files, for Windows 8.1
and Windows Phone 8.1, to have file names that are a bit more consistent with
their 8.0 counterparts.  I.e. "SDL_image-<platform>.<ext>"