Updates to boto to support EC2 IAM roles (security tokens)

EC2 roles are a much more secure way to assign keys and secrets to an instance, and I've updated the elb-presence to support this. Not sure what happens in the case where plain old access keys are used, as I don't use these anywhere in my environment.
hmm... I've updated to default to None as per the [boto documentation for ELBConnection](http://boto.readthedocs.org/en/latest/ref/elb.html). However I'm not sure why that would evaluate to empty string in the first place. Accessing dict.get(...) in python should default to None instead of empty string if the key isn't present in the environment variable.

Having said that, I didn't test this... so willing to accept what you're saying.