Add Selenium test suite to facilitate the testing of the library on different browsers.

[ntoll]

Since sinon mocks away the XHR request in the jasmine based test suite we've noticed a class of bugs related to specific implementation details of the XHR object in different browsers. We need to be able to test that the library works on at least Firefox and Webkit. Selenium seems to be the obvious choice to facilitate this.

[ntoll]

Actually, upon reflection there may be a better / easier way to achieve this. Will add notes to this ticket when I've had time to think about it more.