Jenkins now building pull-requests against apache/accumulo

[Josh Elser]

FYI, I just enabled a new Jenkins job that should see any pull-requests 
opened against http://github.com/apache/accumulo, build it, and share 
the results.

* https://builds.apache.org/job/Accumulo%20Pull%20Requests/
* https://blogs.apache.org/infra/entry/github_pull_request_builds_now
Feel free to make any other changes to the job that make sense. I left 
most of the defaults from the regular hourly-polling job.

- Josh