Added documentation.

I started using Django Gears about a weeks ago. It's awesome, but the lack of docs makes it hard to work with.

This pull request adds documentation to the extent I understand it. I believe it is fairly complete.

You can build the docs with ``make html``. The conf.py assumes ``sphinx_rtd_theme`` is installed. That's easy with pip.

Included with the docs are two Python changes. These make it easier to get start with Gears.

1. Default the ``GEARS_ROOT`` setting to ``STATIC_ROOT``. ``GEARS_URL`` already defaults to ``STATIC_URL``. Mirroring that here makes installation simple.

2. Added ``django_gears.urls.gears_urlpatterns``. This works like ``django.contrib.staticfiles.urls.static_urlpatterns``. It makes it easy to add the serve view without requiring the use of runserver.

Thanks for making this package available. It's a smart way to handle static files.