Add ec2_elb_lb module for managing ELBs

As part of an Ansible implementation project I'm working on, I needed to create a module that not only managed instances on an ELB but actually the presence or absence of the ELB itself. (It's actually important in my particular use case that instance registration be handled separately).

Anyhow, I ended up writing a module to accomplish this which I'd like to hear propose for inclusion. It's a big module that's moderately complicated (probably on par with the other AWS cloud modules), so I expect it might require some discussion etc.

The code and documentation should explain it's behavior pretty clearly. Note that there are a few things which this module does not support, including:

* Healthcheck policy
* VPN / subnet functions

I'll be adding healthcheck functionality for our own implementation so I can update this pull request if need be.

For now I'll open the pull request and would be interested in comments, questions, suggestions, etc.