Use watchify to make builds faster and automatic.

In addition to being an incremental improvement, this also sets us up to browserify Cesium (in order to only pull in the parts we use) along with National Map and to browserify our coming-soon tests, all while maintaining a build that completes in under two seconds.

I had to use a fork of watchify due to [watchify issue 54](https://github.com/substack/watchify/issues/54).