Create documentation (tutorial) and reference (methods and docstrings)

[fccoelho]

We need a sphinx documentation for the API ASAP!

I am marking this as a bug because an API without docs is pretty much useless.

[turicas]

There are not so many methods in the only one class available (PyPLN), all of them have docstrings and the general usage of them are covered in the tutorial.
I think we should not waste time now in this library until we have a REST API on pypln.web, so we can work in a more solid code base (the current code is just a temporary thing). Because of this, I removed the label "bug".

[fccoelho]

It shouldn't require much effort just the setup of a sphinx project for the API, it would then extract the docstrings and generate a minimal documentation. But I'll leave the decision to you since I am not the one doing it.