Remove AVStream.r_frame_rate

It was deprecated in Libav, and can easily not be compiled in. Their reasons are
seemingly valid, so it is tons easier for us to simply remove it from PyAV.

This hopefully fixes #52.
Remove failing asserts due to missing r_frame_rate
Don't log after shutdown by default
"Fix" failing probe tests on OS X with FFmpeg

I don't really know what is going on, but `ffprobe` reports the same values, so
it isn't really an issue with our use of the library.
Compare against ffprobe instead of hard-coded values
Can't use subprocess.check_output for Python <= 2.6