Support data streams with NULL codec.

Spurred from #353 (closed), and others, it would be great if PyAV would support data streams. Those streams appear to be ones with no codec.

Since PyAV is built generally assuming that there is a codec per stream, many things break or even segfault as we start adding the support. It shouldn't be "hard", but it is going to be a big pass over all of the container code to add a ton of if codec checks.