Create an admin mixin for external use.

There are cases where one might have a different base admin class, and
wants to use the search features in the admin as well. Creating a mixin
makes this a bit cleaner.
Add some spacing
Document support for searching in the Django admin
Merge pull request #1084 from streeter/admin-mixin

Document and add an admin mixin
Tests: have Solr test server startup script purge corrupt tarballs

This avoids tests failing if a partial download is cached by Travis
Tests: fix Solr server tarball test

On a clean Travis instance, the tarball won't exist