Close Multiprocess progressbar Pool

The multiprocess option in ProgressBar needs to be closed and joined, to prevent it spinning off into oblivion.

See:
- http://docs.python.org/2/library/multiprocessing.html#multiprocessing.pool.multiprocessing.Pool.close
- http://docs.python.org/2/library/multiprocessing.html#multiprocessing.pool.multiprocessing.Pool.join

This doesn't matter most of the time, but sometimes I get a ton of threads in the background driving me nuts!