Moving to github/pull-request for code review and check-in

As per mailing list discussion, we are going to change the code review and commit process to use github pull request. This would require a few things:

1) incorporate kafka-merge-pr.py from kafka repo to Samza that include the merge steps for github pull requests to Apache Samza git repo
2) update the documentation in contributor corner for the new developer
3) Document the commiter instructions on using kafka-merge-pr.py.