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)