Support for any SQL DB

Brian, this is a monster pull, and not mergeable as is, but I think it's a useful one and would love to hear whether you think it'd be worth my updating it for integration into master.

I've replaced the SQLite DB code with Apache's DBD SQL abstraction layer so that multiple Apache servers using mod_auth_openid can share the same session DB. We've been using this branch at KIXEYE for a few months now to store sessions in MySQL, so that a user accessing a cluster of servers behind a load balancer only has to log in once.

I've also added a test suite with code coverage instrumentation, updated the docs, and fixed a few bugs.

The module is not quite backwards compatible with the original; specifically, tables must now be created before first run with a table maintenance utility (this was due to mod_dbd limitations), and the DB location directive has been subsumed by mod_dbd configuration, but you can still run it with a SQLite backing DB if you've only got one Apache instance.