Adding support for EC2 security groups 

Hi Michael,

First off I just wanted to thank you for your wonderful EC2 library. I've been using it in my own projects and really appreciate the work you've done.

To better support some of my own work, I've appended some additional code to add/modify/delete EC2 security group definitions in clj-aws-ec2.

I attempted to model all the code based on what I found in the library, and I hope you find this contribution beneficial. As I'm not too familiar with github, or the various etiquette around contributions, I hope you forgive any of my mistakes. 

Hope this helps,
    Alan Busby