This plugin builds pull requests in github and report results.

https://wiki.jenkins-ci.org/display/JENKINS/Github+pull+request+builder+plugin

When new pull request occures in project and author of the pull
request isn't whitelisted, builder will ask "Can one of the
admins verify this patch?" and one of the admins must add the
author to whitelist with comment "this is ok to test".

If author of the pull request is whitelisted adding new pull
request or new commit to existing pull request will start new
rebuild.

New rebuild can be started also with comment "retest this please".

Requirements:
github-api plugin (https://wiki.jenkins-ci.org/display/JENKINS/GitHub+API+Plugin)
github plugin (https://wiki.jenkins-ci.org/display/JENKINS/Github+Plugin)
git plugin (https://wiki.jenkins-ci.org/display/JENKINS/Git+Plugin)

Installation:

create github 'bot' user for commenting in pull requests
add the bot as colabolator to your repository

install plugin
go to administration -> system setting -> Github pull requests builder section
set your bot github username and password
add github usernames of admins (will be used for all jobs)
  you can modify phrase for adding users to whitelist (advanced)
  you can modify phrase for retesting (advanced)
  you can specify crontab line click (advanced)
save  


Creating job:

create new job
add github url to "GitHub project" field
select Git SCM
add your github "Repository URL"
into (advanced) "refspec" enter "+refs/pull/*:refs/remotes/origin/pr/*"
into "Branch Specifier" enter "${sha1}"
check "Github pull requests builder"
  you can edit admins for this specific job
  you can change crontab line for this specific job (advanced)
  you can edit whitelisted users (advanced)
save

You are done :)