Add postgres support

[brdweb]

Would love to see support added for postgres or even a database-abstraction layer added.

[flavorjones]

@brdweb We have no plans to add postgres support here at 37signals, but if that's something you're interested in contributing, we'd certainly consider it!

Primarily the database-specific code is for search; and, well, an abstraction for search in Rails is the great white whale and turns out to be remarkably hard to do well across the big three databases.