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.

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.
Merge pull request #1147 from jeremyfelt/grunt

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