Add optional whitelist for users who can retest builds

If the retry_whitelist option is enabled the commenting user must
be on the list or the retest attempt is ignored. The author is always
allowed to ask for a retest though.
Closes #37
Update README
Consider jobs aborted in Jenkins to be finished

This should address #40 by making sure aborted jobs are properly
marked as finished with Mergeatron, letting it know it shouldn't
pay attention to them any more.
Don't run builds for Pull Requests that can't be merged
Move Jenkins token parameter to be project specific