chore(build): implement gulp task for build

Can now be built with `gulp build:amd` or `gulp build:cjs` to build using RequireJS modules, and
CommonJS modules, respectively. `gulp build` is a short-hand which will build using both
module systems.

I would very much like for Traceur to enable builds which don't depend on RJS, and hopefully in
the future, this will be possible.

Closes #5