Use jquery

Okay here goes...

- Removing the hand-rolled stuff in favour of the jQuery
- We're using the same version of jQuery as in alphagov/static
- Removing the stuff about minifying (as no one was using the minified versions and *insert web framework here* does it anyway)
- Tidied up the tests a bit
- Passing in the analytics callback directly instead of using a config object and being explicit about it being a callback
- This feels like we're coupling ourselves to GOV.UK a lot (but as they're the only people we're integrating with at the moment we can solve the problem later)
- Updated the README