README.rst tweaks
README.rst tweaks

This falls out of consistency with http://kivy-garden.github.io/garden.filebrowser/api.html. It may be necessary to create a readthedocs page and refactor README to separate from sphinx-specific rST roles from plain old docutils.

- ``:class:`` in favor of normal quotes
- ``.. code-block:: python`` to get highlighting working
kivy-garden/garden.filebrowser