Allow gerrit status updates to be published via buildsets

I needed to push updates to gerrit per set of builds to keep related information together.  This also allows the verified option in gerrit to work properly.

I found that mail.py contained the needed functionality, so I removed all the unneeded email code and copied over the relevant gerrit code from status_gerrit.py.

The relevant code from master.cfg:
from buildbot.status.gerrit_status import GerritStatusPush
c['buildbotURL'] = 'http://buildbot.url/'
c['status'].append(GerritStatusPush('gerrit.installation.url', 'buildbot-user',
                                    reviewArg=c['buildbotURL'],
                                    buildSetSummary=True))

This code is verified working as of 0.8.6p1.

I understand that this needs to be merged into the existing status_gerrit code (and cleaned up..), but I do not know Python well enough to do so myself with the time I have available.  I also believe buildset functionality may be removed in 0.9.x; I hope a similar implementation will be available to allow this functionality to continue to operate for gerrit status updates.