emcee/interruptible_pool.py: new SIGINT-friendly pool wrapper.

Python's multiprocessing.Pool interacts badly with KeyboardInterrupts,
which is annoying if your parallelized emcee run encounters problems.
This new module wraps the builtin Pool with a shim class that avoids
these problems. It's based heavily on the solution proposed by
John Reese:

  http://noswap.com/blog/python-multiprocessing-keyboardinterrupt
emcee/ensemble.py: default to using InterruptiblePool rather than the stock Pool
PEP8
more PEP8 and documentation
updating changelog and adding @pkgw to contributor list