Sphinx REST api

[boyska]

With this PR we can have HTTP API documentation. However, this requires some little change to the command you use to generate documentation. Please note: just sticking to the current commands does not break anything: you will only miss what I added.

Just tell me how

Here is what you used to do till today:

python setup.py build_sphinx

which is great. If you do it after merging this, you will see

WARNING: sphinxcontrib-httpdomain not installed,you will get no REST API doc
  You might want to do pip install -U pip && pip install '.[doc]'

Despite the warning, there is no error: documentation is still built, but the "HTTP API" page is empty.

Do what it says, and your favourite command will produce HTTP API, too.

The theory

The sphinx extension that does all the magic must be installed. You could do it running pip install sphinxdomain-httpcontrib but that's so unelegant. The right way is specify an "extras set" inside setup.py; doc sounds like a good name for it. After that, you can just install it with pip install .[doc] and everything works. Except that extra set in local directory are only supported in pip >= 7, which is still not very widespread. So pip install -U pip will install it; after that, you can install libreant with the doc extra set.

This will require some small change to readthedocs configuration. This will close #168

[boyska]

Is someone willing to test/merge it? I think this is a coll thing to add to libreant documentation.