Javascript test-related updates

* Added phantom-sahi.js as part of the repo as it seems stale and always required (see [Shane the Hat's post](http://shaneauckland.co.uk/2012/11/headless-behatmink-testing-with-sahi-and-phantomjs/)).

* Added to readme some bits from Shane's post that were relavent, and instructions on how use Sahi with phantom once things are set up.

* (Also added .gitignore to ignore the artefacts produced by running composer.)