Introduced Sphinx based Documentation

To auto-generate the documentation for the ess-player project switch
into the doc directory and run:

  make clean html

Signed-off-by: Lars Kiesow <lkiesow@uos.de>
Merge branch 'sphinx-doc' into master

This will introduce sphinx based documentation generation for the
ess-player project.

Signed-off-by: Lars Kiesow <lkiesow@uos.de>