Use watchify

This commit adds [watchify](https://github.com/substack/watchify) in place of **browserify** when files are being watched. Way faster rebuilds! (10x faster on bigger projects)

From the docs:
> [watchify] ... is exactly like a browserify ... except that it caches file contents and emits an 'update' event when a file changes. You should call w.bundle() after the 'update' event fires to generate a new bundle. Calling w.bundle() extra times past the first time will be much faster due to caching.

Resolves #13