GitLab

Fixes #667

The sphinx version is pinned by readthedocs, but sphinx 1.3.5 is not
available with conda python 3.6. The workaround is to pin the python
version to 3.5 (it doesn't really matter for the docs build).

* Make 'any' the default markup role for Sphinx docs

* Automate generation of reference docs with doxygen and breathe

* Improve reference docs coverage