Implement Watchify for js bundling.

[krambuhl]

Browserify doesn't have a caching mechanism built in, so when running the build-watch build process, javascript edits take 2-4s to completely bundle. Watchify solves this issue by caching and rebuilding only the files that have changed.

This is a straightforward change, but because of how watchify is architected, the gulp task must be configured in a non-gulp-standard way. Whereas the browserify task can call the gulp.task async callback in the standard way, the watchify task continues to run after the gulp.task finished, watching for file changes.