Add Elastic Map Reduce support

[kagminjeong]

(former issue #38)

This branches squashes everything into one commit.

I kept the branch with individual commits as well: https://github.com/richmeyers/right_aws/compare/emr

I tested each action (manually) and we have also been running run_job_flow/describe_job_flows in production for a while.

[konstantin-dzreev]

merged, thanks