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.
Having both `i` and `_i` in the same method is… somewhat horrible. `length` would be a better variable name than `_i`.