JS in external files

This PR extracts the javascript for live-updating build output into its own file.  It also sets up a **really** rudimentary asset pipeline and some jasmine tests for the extracted javascript.

I don't love the asset pipeline (it just concatenates all of the scripts together into main.js, and regex-appends a hash of the result onto the script src to bust the cache) but I couldn't come up with a better way to bust the cache without littering every handler with md5sum code.  I'm not all that familiar with the go ecosystem, so feel free to rip that part out if something better exists.  Grunt is a commonly used tool, but I wasn't sure about bringing in external dependencies.