Support multiple source security groups in non-VPC SG methods

The EC2 API supports authorizing or revoking access from multiple security groups in one call. boto does not currently have parity with the API in this respect.

It's important to be able to make such changes in one call rather than many because of EC2's implementation of security group changes, which appear to full tear down and then reimplement firewall rules, causing a brief network interruption on the order of 10-15 seconds per change. (See http://www.scalehorizontally.com/2012/12/21/aws/ -- thanks to @craigtracey for writing that up.)

I would be happy to add this support to authorize_security_group_egress() and revoke_security_group_egress() as well, if you agree with my approach so far. (Notably those methods don't support multiple CIDR blocks specified at once; I could add that as well.)

Thanks to @jordansmith for advice on this work.