Add ability to navigate serach results with keyboard

This allows you to:

- navigate up and down the list of results with UP/DOWN, or TAB / SHIFT+TAB
- add the selected class by pressing the ENTER key
- clear search with the ESC key
- clear search by clicking outside the search results
- continue to use the mouse as expected, including adding classes and clicking into the search box to edit queries

Selected result (which will be added if ENTER is clicked) is shown in blue, and defaults to the first element.  Elements also turn gray on hover, and can be clicked to be added.  (this mirrors the Google Chrome omnibar search behavior)

![screen shot 2014-07-31 at 1 31 20 am](https://cloud.githubusercontent.com/assets/2433509/3760269/f01dc10a-1873-11e4-9711-9ef0419d430b.png)

Closes #197 
This method doesn't work if you click a child of `'.search-input'` though, because `e.target` ends up being one of the child nodes if you, for example, click some text inside the search results.  It's still doable to traverse up the DOM tree to see if the clicked element is inside that div, but it gets really ugly.