"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
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.
Disable failing asserts due to missing r_frame_rate
First attempt at Python IO
Stop using deprecated flag
Pull file methods off early; writing
Don't log after shutdown by default
Track Python stream position for the user