Name cache busting jar symlinks after the md5 of the jar contents.

This allows us to get away from `Tempfile`, which is giving me grief
on jruby 1.7.8 under hard-to-identify circumstances.  It also makes
better use of permgen ram.
Merge pull request #118 from seomoz/phs/name-cache-busting-jars-after-content-hashes

Name cache busting jar symlinks after the md5 of the jar contents.