Use vanilla watchify/st in place of beefy.

<3 beefy, but this allows us to:

* Disable sourcemaps, which slow the builds.
* Get incremental rebuilds, making refreshes much speedier.
* Potentially disable deduping if required.
* Other messing around directly with browserify.
Handle errors properly
Merge pull request #39 from stackgl/watchify

Watchify