Added swipe view for image diff

As the title says, added swipe view for image diff.

Screenshot:

![Screen Shot 2013-01-27 at 18 25 57](https://f.cloud.github.com/assets/351038/101132/a64c8ab2-68a6-11e2-92d8-ff7dea22978a.png)

![screenshot](http://i.imgur.com/qW4KK.png)