Add support for VPCPeeringConnections.

Added support for VPCPeeringConnections (create / describe / accept / reject / delete) along with corresponding unit tests.

Note: The "delete_vpc_peering_connection" test depends on the accepted-yesterday-but-not-yet-released (as of July 31 @ 2:50p PST) fix for Boto issue https://github.com/boto/boto/pull/2475.