Reboot

Fixes #2, #3, #4, #5, #6.  Changes the interface to more closely match grunt-browserify while preserving the `keepalive` option.  Uses watchify directly, sharing caches across bundles when building multiple files for the same target.

Happy to discuss in smaller chunks if this feels like too much at once.  Also wondering if it makes more sense to merge this project with grunt-browserify to ensure the interfaces evolve together.  Thoughts? 

Depends on substack/watchify#6.