Add Ubuntu 13.10 and 14.04 AMIs

Based on <http://uec-images.ubuntu.com/query/>. AMIs are amd64, instance-store, pv. Let me know if I've mistaken the existing AMI kernels.

Tested with <https://github.com/justincampbell/kitchen-ec2-test/blob/master/.kitchen.yml> against `us-east-1`. I can test against other regions when I have more free time.