Wrapper for 'nose' unittest and code coverage tools.

Summary:
Wrapper for Python 'nose' (http://readthedocs.org/docs/nose/en/latest/)
testing tool.

Test Plan:
Install latest 'nose' v1.1.3. Currently it is available through
Github only (`pep install git+https://github.com/nose-devs/nose.git`).

Create a Python project with following structure:

/package_name/module_name.py
/tests/package_name/test_module_name.py

Write some tests

Run `arc unit`

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Koolvin, zeeg

Differential Revision: https://secure.phabricator.com/D2322