Modularization and Plugin infrastructure

[moschlar]

In order to simplify extension and to support domain specific tasks (e.g. source code analysis which is dependent on the used language or user-supplied test validation scripts etc.), a plugin infrastructure would be very useful.

It would be required that all currently internal specific tasks are already factored out as some kind of built-in plugins. Plugins must have an administration interface where they can be activated/deactivated and configured.