Update documentation generation script for Travis

This pull request introduces several changes to the Travis script:

* Use newly added _CMake_ options
    - to produce tutorial files with `.php` extension
    - to force _Doxygen_ use long descriptive filenames
* Push generated tutorials and advanced articles to the 'documentation' repository
* Use `--amend` option to avoid excessive storage usage (as discussed in #584)

Before merging this pull request we need to wipe out the 'documentation' repository to get rid of thousands of megabytes generated in the past months. I'm not sure if there is a way to do this other than delete the GitHub repository completely and recreate it.