Adding -availability_zone param for create_subnet()

It looks like VM::EC2::REST::subnet::create_subnet() can accept an
-availability_zone parameter, but the VM::EC2::VPC::create_subnet()
convenience method doesn't seem to have a way to pass it through, so I
added one.
Merge pull request #25 from hakamadare/availability_zone-param-for-create_subnet

Adding -availability_zone param for create_subnet()