Screenshot of element and checking diffence between images

Part of the dalek PR https://github.com/dalekjs/dalek/pull/112

To dalek-driver-native was added **Image** lib that support necessary for changes in dalek operations. **Image** lib relies on [node-pngjs](https://github.com/niegowski/node-pngjs) lib, that is simple pure js lib for work with png files.


I was inspired by @stoyanstefanov post http://www.phpied.com/css-diffs-2/

I'm quite new in github and open source contribution, so please let me know if I'm doing something in wrong way.