`utilities.py:_open_for_write` no longer has a race condition

[rawrgulmuffins]

Fixes #117

Changes

• Added Pathlib2 dependency so that Python2 has the same interface as Python3 for file system manipulation.
• Changed _open_for_write to use Path.mkdir for cache file creation to avoid race conditions.

Testing

% nosetests test/unittest
nose.plugins.cover: ERROR: Coverage not available: unable to import coverage module
................................................................................
----------------------------------------------------------------------
Ran 80 tests in 1.818s

OK

NOTE

In a perfect would I would prove that there's no race condition using injected locks to stall the main process until the cache directory is created but I don't know how to do that with multiprocess.

[rawrgulmuffins]

Looks like in 3.4 Path.mkdir doesn't have the exists option. Switching to pathlib2 if running on Python3.4.

[sk-]

Thanks for the changes.

[sk-]

Version 0.1.2 was uploaded to pypi.