Postgres hstore support

Sending this to you since @nulldb doesn't seem to be active.

This passes simple hstore-in-the-schema tests, and works against my Rails 4 postgres codebases. However, it begs the question, how should nulldb support db-implementation specific columns types baked in to activerecord?

This pull request pretty much hard-codes support just for hstore in but I plan on refactoring into something more dynamic/configurable. I have a few different ideas for the refactor, but I'm interested in your opinion on what approach to take.