Block LocalMRJobRunner until fewer subprocs

Pretty proud of this one. I thought it was going to be a real cunt :)

This allows you to limit the number of subprocesses which will be
spawned by the runner so that you needn't worry about the box being
overrun by resource-hungry jobs.

Was about to dip into Queues and such, but this seemed the far simpler
and equally-effective option.