Adding option to specify IAM job flow role to use with EMR cluster

This fixes issue: https://github.com/Yelp/mrjob/issues/856

EMR can take in an IAM role for the job flow. Restrictions placed on the IAM role will then be placed on the machines of the EMR cluster too, helping organizations use these roles for additional security.

(see http://docs.aws.amazon.com/ElasticMapReduce/latest/DeveloperGuide/emr-iam-roles.html for more info on using IAM roles in EMR)