Added parallel migrations

This is a port of propelorm/Propel#777 to Propel2, without the backwards compatibility. This feature is request in #145. Basically it's just tracking of all executed migrations instead of the latest one.

Only problem is that I'm unable to write tests. I need to call `$migrationManager->setConnections(..)` in the tests but I'm not sure where to get the connection settings from. If someone could help me with this, I'll try to write some tests. 

I would really like to see this feature in Propel2 because it's really hard to use propel migrations if you have multiple feature branches.