docs: Use 'None' and explain why.

An example should always show the 'best' or recommended usage, not a
possible one and then in a note document the preferred usage.
docs: Add a trick for supplying partial config.

I had to figure this out myself but it's been an incredibly useful
snippet to minimise the amount of configuration a user needs to do. This
is mostly useful for plugins that have more than a few configuration
options.
docs: Update instructions on testing a plugin:

  * Goodbye unittest, hello py.test:
    * Goodbye horrible `setUp` tasks and other bloat
    * Hello pretty `assert` statements
    * Remove other unittest code bloat
    * Change the command invocation for the tests to `py.test`
    * Use `pytest-pep8` plugin for PEP8 support
    * Start the classes with Test to adhere to Python test discovery
      rules
  * Update the .travis.yaml sample:
    * Yapsy fixed their name, so pypi and setuptools are happy now
    * Add testing on Python 2.7 and Python 3.4
  * Add explicit `.. code-block:: python` everywhere. This causes IDE's
    to turn on syntax highlighting for those sections and is more in
    line with the rest of the documentation.