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, but they all pass - the DB subnet group is created so quickly that I can't issue a request fast enough to discover its intermediate state. The docs and a fair amount of googling have turned up no answers, but we have an AWS enterprise support account; I'll ask them. In the meantime, can this be merged, as the lack of intermediate states is fine in practice?