Travis CI integration

Just having Travis CI build every commit is a huge plus, so I think we should add support. Here is a working configuration. Travis is able to create a binary. It probably wouldn't run, since it mixes different libav-versions in one binary (system OpenCV and newer libav from ppa), but since Travis uses Ubuntu 12.04, I couldn't come up with anything better at the moment.

Maybe I build my own ppa later, but don't hold your breath ;)