Ignore jetty/ except the config.

Jetty is a little snafu here, and here's what I've been able to deduce:
  1. First, jetty/solr/development-core/* and jetty/solr/test-core/* were pulled
     in as a solr-only checkout of jetty.
  2. Then, the conf was copied to jetty/solr/production-core/*
  3. But then 'jetty' directory was added to .gitignore before jetty/solr/production-core/conf/*
     was committed, so it has never been tracked, afaik.
  4. And finally, the rake tasks that are supposed to result in a working jetty config
     do not work; results in java errors.

TODO: Rake tasks should result in a working jetty config. Until then, storing
working configs as part of repo.