Merged, pushed and email sent.

Looks good to merge!

I took care of your comments. Can I merge this?

I made some comments. Once this is merged, I think this is worth a short email
to the public mailing list, as it changes how most people are supposed to build
the planner.

I made a pull request at https://bitbucket.org/jendrikseipp/downward/pull-requests/76 .

We would like to let build.py use all available cores by default. This makes it 
easier to let the buildbot use the correct number of cores depending on the 
machine it is running on.