update github repository url
beamer: clean *.vrb files

*.vrb files are created for fragile frames, that you have to write in
order to have verbatim in frame, like this:

    \begin{frame}[fragile]{Hello, world!}
      \begin{verbatim}
        printf("Hello, world!\n");
      \end{verbatim}
    \end{frame}
Makefile: suffix archive with -src
Makefile: add a dist target

This target is used to archive the final PDF file and (if needed) the
src/ directory containing the listings.
Makefile: add distclean target
Makefile: hide which stderr