Reduce code duplication in tests

Create default response with status code 200 and rename session to request.
Move fixtures from tests.py to fixtures.py
Add fields argument to update methods
Reorganisation in api, client and models
Change README to reStructuredText

All documentation should be in .rst so that all of the doctests are run by
nose.
Initial attempt at sphinx documentation
Reorganise documentation into a single page
Move module test setup to cosm.__init__.py

And request_handler to fixtures.py.  It would be nicer if we could move the
test code completely out of scope, probably requires subclassing the test
runner.
Make url a public method on managers.

base_url is now a property that can be set or looked up on parents.
Documentation for Feeds
Add example script (currentcost2cosm.py)