Add MonoTouch support to PlatformAttribute

[Charlie Poole]

Both[Platform("MonoTouch")) or [Platform(Exclude="MonoTouch")] should work properly.

Here's the code supplied by Sebastien to detect it:

   Type.GetType ("MonoTouch.UIKit.UIApplicationDelegate, monotouch") != null

Note that Type.GetType("Mono.Runtime") may be null, since the type can be removed by the linker.

It would also be great if we could add detection of the actual device!

Charlie