uiimage#masked, create a masked image from input mask

masked method for UIImage that takes a input mask image parameter. This will create and return masked output UIImage. 

I've created a [gist to show how it'll work](https://gist.github.com/4532889). You can use the input images I've attached.

![kitten](https://f.cloud.github.com/assets/58212/65651/32e69cae-5e86-11e2-9e00-2b9580cdd891.png)
![mask](https://f.cloud.github.com/assets/58212/65652/32ed66ba-5e86-11e2-891e-d1707279493e.png)

This should give you something like this:

![Screen Shot 2013-01-14 at 10 13 41 PM](https://f.cloud.github.com/assets/58212/65671/04917a08-5e87-11e2-9d7c-8631588f82f7.png)