Implement aws_db_subnet_group resource

Depends on mitchellh/goamz#74. Related to #28.

Also adds a `db_subnet_group_name` parameter to `aws_db_instance`.

I'm new to Go, and mostly fumbled my way through this PR by copying and adapting `aws_db_security_group`; sorry if I screwed up any idioms.
Yes, I imagine this won't work as is, due to this. If we run the acceptance tests it should give us the various states. Are you familiar with how to run those?