Phantom CSS screenshot comparison

Compares screenshots of article page at 3 different breakpoints.

To run do:
``cd integration-tests/phantomcss``
``phantomjs ./demo/guardian.js``

or 

``./screenshotcompare.sh`` from the project root

Warning: PhantomJS [doesn't support webfonts](https://github.com/ariya/phantomjs/issues/10592) hence the screenshots looking a bit strange. Switching to [slimerjs](http://slimerjs.org/) in future would fix this