VPC Commands

Hi --  First, really like the erlcloud package, it's been enormously useful.  For some of the work we've been doing I've needed access to the VPC commands.  I've taken the liberty of hacking those into the erlang_ec2 module and at the same time updating a few of the existing commands to understand a newer API (specifically version 2012-10-01) where the addition of the VPC feature has changed the way the options the command accepts.  (create_security_group is a good example of this: now there are both EC2 instance and VPC security groups; to create the latter requires GroupId instead of GroupName.)

I've tried to stay in the spirit of your API -- specs provided, config is optional, and so on to make it easy enough to incorporate these changes.  That said, my primary objective (i.e., what I get paid for!) has been to get an AWS infrastructure up and running, so regression testing certainly isn't complete and I haven't implemented everything, just what was needed.

I'm sending the pull request because some of the functionality might prove useful but understand completely if you'd rather not merge my changes in.  Thanks for maintaining such a useful package!

k.