EC2 Subnet added, with DescribeSubnets support initially.

No live tests currently - since we only have subnets (and for that
matter, only DescribeSubnets and not Create or Delete) so far, and not
VPC, we can't be sure there'll be a subnet to list against in the
first place. That's still to be done.

Note - I have written a live test verifying that list() and filter() work correctly (filter for both finding something and not finding anything) but they're against my account which does have existing subnets.