chrome as optional driver

Adds the ability to run tests in chrome using the property -Ddriver=chrome

*Gotchas*
To run this locally you will need Chromedriver in your PATH https://code.google.com/p/chromedriver/
For this to work in teamcity, chrome and chromedriver will need to be in the PATH (I am talking to websys about the best way to do this)
clearing local storage does not appear to work in Chrome, this is http://code.google.com/p/chromedriver/issues/detail?id=153 to stop it throwing an exception I've had to put some conditional logic around running this, this causes a few tests to fail which rely on clearing local storage. I couldn't find a workaround for this.