[MRG] supporting the new Python 3.4 multiprocessing API

This is an early pull request to add support the new context-based Python 3.4 multiprocessing API.

This is a both a fix for #98 and #87.

<del>Note that to run the tests with `nosetests` you will need to add a symlink named `nosetests.py` in the same folder as a workaround for [Python issue19946](http://bugs.python.org/issue19946).</del> This issue has been fixed in the dev branch of Python.

<del>Note: I will fix the hashing / save_global bug #97 separately as it's independent of multiprocessing and rebase that branch on top of master once fixed there.</del> (Done)