add jenkins-pull-requests-build script

Move our Jenkins pull requests build script out of Jenkins' database and
into Git so that we can have greater visibility on future changes.

Going forward, we will modify the teuthology-pull-requests Jenkins task
to only run this script.

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>
Merge pull request #190 from kdreyer-inktank/jenkins

add jenkins-pull-requests-build script