Add Sphinx documentation

[benoitbryon]

What about having a Sphinx documentation? Not "just a README"? Here are some advantages:

• publish docs on readthedocs.org
• have a shorter README (summary + 1 example + links). Example here: http://django-downloadview.readthedocs.org/en/latest/ seen also as https://github.com/benoitbryon/django-downloadview/
• explain usage by topics, in separate pages: how to write a decorator, how to test a decorator, how to install decorum...
• add documentation about project itself: vision, alternatives, history (changelog)... Examples here: http://diecutter.readthedocs.org/en/latest/about/ or http://django-anysign.readthedocs.org/en/latest/about/
• have doctests! See also #8

[benoitbryon]

@zeekay: I volunteer to setup Sphinx documentation (see also #8). But, it is quite a big change, so I'd prefer you agree before I start. I mean, tell me whether you would consider a pull-request with Sphinx or you definitely want to keep docs in a single README ;)

[zeekay]

If you love Sphinx, I'm down. I've used it on larger projects...seems a tad overkill here (unless functionality significantly expands). I'll defer to you, as you'd be integrating it.

Plus I can always use whatever you add as a template for my other projects, so that's a win for me ;)