Move driver registration to a sub-package.

I don't really expect this to be merged since it's not backwards compatible, but just wanna open it up for some discussion.

Currently, if you want to use `pq` as a library and wrap it with your own postgres driver (say, to increment statsd counters when opening/closing connections), you have to register your own driver under a different name or database/sql will complain.

Moving the driver registration to a sub-package alleviates this problem.