Mock out the full AWS EC2 networking layer

This adds mocking for the relationship between instances, network interfaces, subnets, and VPCs.  It also includes subnet validation so that interface and subnet addresses must be within the subnets of their parents (and can't conflict with other subnets).  I apologize for the large size, I felt it was better to submit something complete so that it would actually test well.  This also represents about what I could get done in a day.

I also apologize for a handful of white space changes.  My editor removes trailing whitespace. ;-)

Please let me know if you have any feedback!