Screenshots assertion functionality into the test runner.

Sometimes it is really hard to assert all parts of the view in complex application and asserting against some previous captured as image state is really helpful, especially since `UIAutomation` supports screen capturing. 

I have added such solution that relies on `ImageMagick`, but this dependency shouldn't be a problem because it can be installed through the `Homebrew`. 

Currently this feature provides:

1. Assertions based on per [channel metrics](http://www.imagemagick.org/Usage/compare/).
2. Diff images generation ( [example](https://dl.dropbox.com/u/12840562/test.png) ). 
3. Provides formatted output that is supported by current output formatters (XUnit included).
4. Also added UIAutomation helper method for capturing only application window. Since status bar is not a static element of the screen, it can add unnecessary differences into images.