Add Postgres support

[wbotelhos]

@petergoldstein could you rebase and try again, please?

[petergoldstein]

@petergoldstein So the way you reorganized requires much more than a rebase. Some of it doesn't quite make sense for a multi-db setup (for example, you don't want a dedicated mysql.rb file). I can pull it together, but it's a larger change than just a rebase.

[petergoldstein]

Closing this in favor of #12 , which is rebased and restructured for code changes.

[wbotelhos]

It was already there, I've just refactored to expose the hard dependency.

I recommend the following:

1. Save your diff;
2. Reset hard to the root good;
3. Re-apply the diff manually;
4. Force the branch.