[PR] Add Gruntfile.js and package.json for local builds

This is pretty much a direct copy of the PR in #971, which I think still does the trick, but uses the `build` directory.

I've changed the Grunt build directory to use 'local-build' instead so that it does not conflict with the build directory used by the project to provide these files.

I attempted to match the JS concatenaton done in Builder.py, and the local build has worked as expected so far.

Thanks for this project!