Add support for trial -j option

trial -j/--jobs is only supported in Twisted>=12.3.0
Move jobs setup to start method so that trialJobs can be Renderable
Add documentation and fix failing assertion
Rename trialJobs to jobs
Conform to method-naming style
Remove changes to progressMetrics and use a single OutputProgressObserver for all log files
Merge branch 't2421-trial-jobs' of git://github.com/iffy/buildbot
fix pyflakes
Merge branch 'master' of github.com:buildbot/buildbot