Add documentation generated by Sphinx

Move advanced documentation from the README into the docs/ folder. This makes it easier to navigate.

The structure of docs/ is such that it can be turned into a nice web page using the sphinx program. To generate it, run python setup.py build_sphinx.

The documentation is automatically uploaded to pytaskfarmer.readthedocs.io.

The contents of the README have been slimmed down to the bare minimum. More advanced topics are covered in docs/ to reduce the amount of documents that need to be kept up to date.

Several of the docstrings (ones included in the sphinx documentation) have also received updates.

• Fixed to the text and clarifications.
• Better respect of the 80 column limit.
• Use of numpydoc syntax.