Adding Reader.duration_read and Reader.duration_remaining
Minor README changes around v0.5.0 preview notes
Changing Reader and Writer attribute names from samples_<foo> to sample_frames_<foo>

This is a more accurate name for what these attributes return.
Renaming Info.sample_count to Info.sample_frame_count, which is more accurate
Renaming Duration.sample_count to sample_frame_count, which is more accurate
Expanding Duration tests to also test sample_frame_count and sample_rate attributes