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

(see http://docs.aws.amazon.com/ElasticMapReduce/latest/DeveloperGuide/emr-iam-roles.html for more info on using IAM roles in EMR)
Merge pull request #867 from anusha-r/iam-role-1

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