Improve BuildBot MailNotifier improve

Current emails which BuildBot sends are not informative, because they do not allow to understand what happened with the build. That is why MailNotifier service should be customized.

The notifier should send emails for following reason:
1. The build has failed or completed w/ warnings;
2. The build has completed w/o warnings and the previous one wasn't success (failed or have warnings).

The mail should have following information:
1. The step where BuilBot has failed;
2. Warning diff by steps: which warnings are gone and which warnings are new.

The example of MailNotifier can be find here: http://buildbot.net/buildbot/docs/0.8.1/MailNotifier.html