Limiting number of multiprocessing threads/cores

[astrobokonon]

This is a rehash of something I put on the old trac system. It's a simple way to limit the number of spawned processes in the multiprocessing module via a simple keyword argument, and it's worked great for me on a multiuser machine. I think I've added the documentation to the right places in the two changed files already.

[perone]

Thanks man, I'll take a look soon as possible !

[erikreed]

This was fixed in #58; also duplicate of #23.