add StorageType as an RDS option

First off, an apology.  I'm not familiar with Go and I wasn't able to figure out how to compile and run the tests.  I'll continue to bang away at it, but if you wanted to add a `CONTRIBUTING.md` document, that would be welcome :)

This PR is, in part, to address hashicorp/terraform#439.  While I'm figuring out the build chain, I wanted to throw this over the fence.  I hope you don't mind.