First pass at analytics

Part of #70 

* Adds a jshint file for linting (thanks @theresaanna)
* Adds an `ANALYTICS_ID` setting, which should be overwritten (via `local_settings.py` in production)
* Adds this setting to all current templates
* Includes analytics JS setup in base template
* Moves JS for agency autocomplete out of the template into a separate file
* Triggers custom events when a) selecting an agency and b) clearing your choice (presumably your search terms didn't get you want you wanted)

I didn't see a good way to connect a "no results" event, but it's partially captured in event b.
My bad; should have looked it up rather than trying to remember...