Automatic JS minification

Issue #159.

This adds some steps to the paver tasks for each book that minify most of the Runestone JS files using Google's Closure Compiler.

I had to add the closure-compiler.jar as a local file instead of using their web service API because we have enough files that their API rate-limiting is triggered if we run a paver task too often.
This introduces a dependency on Java on the server, but that seems to be okay as Webfaction has Java installed already.

The minification process defaults to off, controlled by the `minify_js` boolean in `pavement.py` or `paverconfig.py`, if it's present.