Add Doxyfile for generating documentation

 - Everything in ./src is included.

 - EXTRACT_ALL is set, i.e. even methods and attributes without
   documentation strings will appear in the documentation.

 - Only HTML output is generated to ./docs.

The resulting documentation should be pushed to gh-pages, which will
appear on http://kitrokopter.github.io/API-Application/

This is done like this:

    # Initial setup
    rm -rf docs # if necessary
    git clone -b gh-pages git@github.com:KITrokopter/API-Application docs

    # Generating the documentation and putting it online
    doxygen
    cd docs
    git add .
    git commit
    git push origin gh-pages