Mention benefit of security in transport with Postgres (closes #23).
Allow specifying job_class as a string, for use when queueing jobs for another process.
Fix outdated fact in readme.
Add Que.enqueue, for a cleaner interface when specifying a job class as a string.
Add specs to ensure that the examples in the customizing_que doc don't break.
Add Que.db_version to easily check what the current schema version is.