Add recipes for nose2 and nose2-cov.

nose2's requirements file is overly specific, so when building we must
use six 1.3, but for actually running nose2, the latest version (1.6.1)
works fine.
Add argparse and unittest2 requirements for Python 2.6
Merge pull request #105 from dan-blanchard/add_nose2

Add recipes for nose2 and nose2-cov