Add doctest support

I don't really know that you are actually hosting nose-machine or just cloning from Google Code but I love github so can you pull my request?

Execute ``nosetests`` with ``--with-machineout`` and ``--with-doctest``
options to see the machineout doctest.

By the way there is [fungusakafungus-nose-machineout](http://code.google.com/r/fungusakafungus-nose-machineout/) which support doctest but actually it doesn't work so I create my own and it works with nose v1.1.2