Block LocalMRJobRunner until fewer subprocs

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.
Merge pull request #16 from duedil-ltd/feature/local-subproc-limit

Block LocalMRJobRunner until fewer subprocs