replace read the docs by github-pages

[Stephan Kuschel]

Idea:

• documentation (html and pdf) is forced-pushed to the gh-pages branch of the repository by travis-ci, on every master branch which has passed the tests.

Benefits:

• No commercials on read-the-docs
• failing doc builds are easier detected.
• Full access to page creation (styles, links,...)
• New postpic webpage would then be: https://skuschel.github.io/postpic

[Stephan Kuschel]

Completed by #259
Still needs to be updated manually, which is probably not too bad, as code changes also may require to recreate the documentation .rst files generated by sphinx-autodoc.