Add auto update javascript

Make search results auto update when facets are changed on the finder.

This pull request does the following:

- Lifts the js from /alphagov/frontend/live_search and rewrites it using prototypes to match the style of the other js in this project.
- Modifies the js to work with this DOM and mustache config + updates tests.
Ok I've emailed Simon Kaplan from the content team for a suggestion of a better error message. OK to merge and then I'll update when he get's back to me?