Support parallel builds with GC3Pie (#1006, WIP)

This is meant to be the implementation of #1006.

It's currently work-in-progress and in very early stages too.
there has to be a parent class `JobFactory` which has all methods with a `raise NotImplemented` (to become the de-facto API). we can then determine all jobfactories by searching the subclasses, so no need for harcoded maps, anyne can just add their own by putting it in the pythonpath (e.g. like easyblocks)