Run tests in various browsers

You'll need BROWSERSTACK_USERNAME and BROWSERSTACK_KEY values set in env. Add to the `.travis.yml` file where I've marked. You'll need to secure the values using: http://docs.travis-ci.com/user/build-configuration/#Secure-environment-variables

Currently there's a couple of small issues:

```
Launching 9 workers
[Windows 8, Internet Explorer 10.0] Tracebacks:
can call superclass constructor
can access superclass methods
Inheritance
[Windows 8, Internet Explorer 10.0] Completed in 36 milliseconds. 27 of 28 passed, 1 failed.
[Windows 8, Internet Explorer 10.0] Screenshot: https://s3.amazonaws.com/testautomation/cd383cc748f2ae9629fe7810a553a0a29b84e6c5/js-screenshot-1404815840.png
[Windows 7, Internet Explorer 11.0] Completed in 137 milliseconds. 30 of 30 passed, 0 failed.
[Windows 7, Internet Explorer 11.0] Screenshot: https://s3.amazonaws.com/testautomation/8195d2087f997c0a2ce0b04c061c12236f303bab/js-screenshot-1404815849.png
[Windows XP, Internet Explorer 6.0] Tracebacks:
null
[Windows XP, Internet Explorer 6.0] Completed in NaN milliseconds. 0 of 1 passed, 1 failed.
[Windows XP, Internet Explorer 6.0] Screenshot: https://s3.amazonaws.com/testautomation/e2e868e51cc51f8f2ecb4cf5d4ca87c9e401e1fe/js-screenshot-1404815877.png
[Windows 7, Internet Explorer 9.0] Tracebacks:
can call superclass constructor
can access superclass methods
Inheritance
[Windows 7, Internet Explorer 9.0] Completed in 24 milliseconds. 27 of 28 passed, 1 failed.
[Windows 7, Internet Explorer 9.0] Screenshot: https://s3.amazonaws.com/testautomation/43d4a235a648d73afbfe48c7a7f3b98a897e5c78/js-screenshot-1404815898.png
[OS X Mountain Lion, Chrome 35.0] Completed in 28 milliseconds. 30 of 30 passed, 0 failed.
[OS X Mountain Lion, Chrome 35.0] Screenshot: https://s3.amazonaws.com/testautomation/d417369b509ddf3047ad2bc0582ba4e2ef81a3d9/js-screenshot-1404815920.png
[Windows 7, Firefox 30.0] Completed in 33 milliseconds. 30 of 30 passed, 0 failed.
[Windows 7, Firefox 30.0] Screenshot: https://s3.amazonaws.com/testautomation/5493d453bccc682bb2d3fb5168c01ed918ac27b9/js-screenshot-1404815947.png
[Windows 7, Chrome 35.0] Completed in 51 milliseconds. 30 of 30 passed, 0 failed.
[Windows 7, Chrome 35.0] Screenshot: https://s3.amazonaws.com/testautomation/4057788e304394320aa7cc70c819680b836511c5/js-screenshot-1404815970.png
[OS X Mountain Lion, Safari 6.1] Completed in 25 milliseconds. 30 of 30 passed, 0 failed.
[OS X Mountain Lion, Safari 6.1] Screenshot: https://s3.amazonaws.com/testautomation/3648f32c8ce0190363775ea5df95f2d38d4a2ea6/js-screenshot-1404815992.png
[Tue Jul 08 2014 11:42:24 GMT+0100 (BST)] [Runner alert] Worker inactive for too long: OS X Lion, Firefox 30.0
All tests done, failures: 4.
Exiting
```