new review bots factory-source and maintbot

factory-source checks if submissions contain sources already accepted in
Factory. maintbot makes sure maintenance submissions are authored by the
Factory package maintainer.

Both are based on a new generic ReviewBot.py class that can serve as
common framework for review bots.
Merge pull request #266 from lnussel/master

check script that makes sure packages are submitted to Factory
Announcer: Factory ISOs are now called Tumbleweed
Merge pull request #270 from DimStar77/factory-package-news

Announcer: Factory ISOs are now called Tumbleweed
Add into into the ignored set the packages downloaded