Add sphinx documentation

[drix00]

I add sphinx documentation to the project. I use the README.rst for the main documentation and add the API Reference, which is generated using sphinx-apidoc.

Thing to do before we merge with master:

• [X] add Read the Docs for this branch: http://pyxray.readthedocs.io/
• [X] applied a theme: Read the Docs
• [X] add version automatically in conf.py
• [X] add Read the Docs badge

[codecov-io]

Codecov Report

Merging #12 into master will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #12   +/-   ##
=======================================
  Coverage   84.13%   84.13%           
=======================================
  Files          18       18           
  Lines        2779     2779           
=======================================
  Hits         2338     2338           
  Misses        441      441

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