examples: added example which uses decodebin

        * examples/Makefile.am: added example to a build list
        * examples/all_media_player.cc: simple application, which can play
          all audio/video formats (if specyfic plugins are installed)
          using decodebin. This example shows, how to use decodebin,
          dynamic pads, and how to use non-static class methods in signal
          handlers (sigc::mem_fun).