PARQUET-58: Add PR merge tool

This is a copy of the merge tool from incubator-parquet-mr. I've used it to merge a PR already, so it works.

Author: Ryan Blue <rblue@cloudera.com>

Closes #4 from rdblue/PARQUET-58-add-merge-tool and squashes the following commits:

b06c32c [Ryan Blue] PARQUET-58: Add PR merge tool and docs.