SPARK-1648 Support closing JIRA's as part of merge script.

Adds an automated hook in the merge script that can close the JIRA,
set the fix versions, and leave a comment on the JIRA indicating the
PR in which it was resolved. This ensures that (a) we always close JIRA's
when issues are merged and (b) there is a link to the pull request in every JIRA.

This requires a python library called `jira-client`. We could look at embedding this
library in our project, but it seemed simple enough to just gracefully disable this
feature if it is not installed. It can be installed with `pip install jira-client`.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #570 from pwendell/jira-pr-merge and squashes the following commits:

3022b96 [Patrick Wendell] SPARK-1648 Support closing JIRA's as part of merge script.