Add a new option '--export-job-name'
Export MRJOB_JOB_NAME if export_job_name is on
Test --export-job-name both on and off
Fixed indentation of example in README
Merge pull request #921 from zonca/patch-1

Fixed indentation of example in README
Merge pull request #916 from smallredbean/export_job_name_to_env_var

Export job name to env var in order to distinguish tasks sharing the same emr job flow