Add unit tests, starting with the Item api methods

Running the tests requires mock and nose. To install them:

    pip install mock nose

To run the tests, run `nosetests` from the repository root.

I avoided touching the pypodio2 files where possible, but areas.py was
failing because Item.update's indentation wasn't consistent with the
rest of the file. I reindented that method, but didn't make any
functional changes to the API itself.