PARQUET-3: tool to merge pull requests based on Spark

given a pull request id on github.com/apache/incubator-parquet-mr this script will merge it
requires 2 remotes: apache-github and apache to point to the corresponding repos.
tested here (pretending my fork is the apache remote):
https://github.com/julienledem/incubator-parquet-mr/commit/485658a5b7654198ab1fcc77e2b850ee12999491

original tool:
https://github.com/apache/spark/blob/master/dev/merge_spark_pr.py

Author: Julien Le Dem <julien@twitter.com>
Author: julien <julien@twitter.com>

Closes #5 from julienledem/merge_pr_tool and squashes the following commits:

f719846 [Julien Le Dem] rephrase 'apache credentials'
faab516 [Julien Le Dem] Create README.md
ab3b8fa [julien] tool to merge pull requests based on Spark