Migrations should be reversible

Ugh, Darren is so annoying. Why waste his time and ours to pester us and our project?

I've never jumped into anybody's project on Github and come out of it without gaining something myself.  Seeing the hstore thing in this project... it flipped a switch in my brain, and suddenly I had the motivation to replace Mongoid in one of our projects with Postgres.  Took a day of TDD, but we're able to query our dynamic data in Postgres in a similar fashion to MongoDB.  

I used the instructions in the ```activerecord-postgres-hstore``` project, and during code review I noticed that my migration looked different than yours.  So I'm sharing back.