Add EMR support

[tylersouthwick]

No description provided.

[coveralls]

Coverage decreased (-0.5%) to 52.19% when pulling 4f29082 on tylersouthwick:emr into fa05312 on MonsantoCo:master.

[bkrodgers]

We do have an AmazonTag class already that could be used here instead of the new CloudformationTags. It does contain an optional PropagateAtLaunch parameter that doesn't apply in this case (it's for ASGs), but we're already using it in other places where it doesn't apply -- it defaults to None. I'd still say use it here though, and if we want to clean that up we can split it out in the ASG implementation.

[bkrodgers]

Thanks for the quick change (and for all your contributions)! LGTM