Adds postgresql support.

Hey @emerleite 

Thanks for this package. We're setting up `node-database-cleaner` in our nodejs tests. I noticed there wasn't any postgresql support, so here's a pull request for it.

I'm not a big nodejs guy, so I modeled everything after the mysql implementation. Let me know if there's anything you'd like me to change. It'd be great to get this into master for anyone else that comes along.