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 the docstring for Item.update didn't have the same
indentation as the rest of the method. I didn't make any functional
changes to the API client itself.
Add support for Stream methods.

Includes some more basic tests to make sure the API methods are properly
hooked up to httplib2.