Add support for nosetests multiprocessing plugin.

Add support for nosetests multiprocessing plugin.

See
  python-nose/doc/doc_tests/test_multiprocess/multiprocess.rst
for an explanation of the plugin and its peculiarities.

On a 16-core SMP machine, the test suite now runs with:
  Ran 27 tests in 58.077s
vs the old:
  Ran 23 tests in 69.325s

The extra tests are repeats, probably having to do with a
split-vs-share issue that I haven't tracked down yet.  The tests could
be faster still if we used the subproc manager by default for Python
>= 2.6.  I'll do that next.