DL-16: Add script to merge GitHub PR.
DL-16: Add script to merge GitHub PR.

Fetch a list of open Pull Request from GitHub, and merge them into the
repository. If the JIRA credentials are exported, the issues related to
the PR are updated with a comment and closed.

This script is a modified version of the one created for the spark
project.

```
./dl-merge-pr.py
Which pull request would you like to merge? (e.g. 34): 2
Commit title [DL-4: Add NOTICE file.]:

=== Pull Request #2 ===
PR title	DL-4: Add NOTICE file.
Commit title	DL-4: Add NOTICE file.
Source		franckcuny/fcuny/DL-4
Target		master
URL		https://api.github.com/repos/apache/incubator-distributedlog/pulls/2

Proceed with merging pull request #2? (y/n): y
From https://github.com/apache/incubator-distributedlog
 * [new ref]         refs/pull/2/head -> PR_TOOL_MERGE_PR_2
From git://git.apache.org/incubator-distributedlog
 * [new branch]      master     -> PR_TOOL_MERGE_PR_2_MASTER
Switched to branch 'PR_TOOL_MERGE_PR_2_MASTER'
Do you want to validate unit tests after the merge? (y/n): n
Enter primary author in the format of 'name <email>' [Franck Cuny <fcunyapache.org>]:
Enter reviewers [Sijie Guo <sijieapache.org>]:

Merge complete (local ref PR_TOOL_MERGE_PR_2_MASTER). Push to apache? (y/n): n
Okay, exiting
Restoring head pointer to fcuny/DL-16
Switched to branch 'fcuny/DL-16'
```

Author: Franck Cuny <fcuny@apache.org>

Reviewers: Sijie Guo <1f0a072942b5a6a61c5ea946a8ff638685cedd4a@apache.org>

sijie/incubator-distributedlog,sijie/incubator-distributedlog,sijie/incubator-distributedlog,twitter/distributedlog,twitter/distributedlog,twitter/distributedlog,sijie/incubator-distributedlog,twitter/distributedlog,sijie/incubator-distributedlog,twitter/distributedlog