Add test utility for generating Jar files with compiled classes.

This was requested by a few different people and may be generally
useful, so I'd like to contribute this and not block on a different
PR for it to get in.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #326 from pwendell/class-loader-test-utils and squashes the following commits:

ff3e88e [Patrick Wendell] Add test utility for generating Jar files with compiled classes.