Support playing sound from stereo files
Stop trying to read from audio file if no data is available

As soon as libsndfile has finished reading a file it no longer returns
any data. This can send the `render` method for `FileRenderer` into an
infinite loop. This way if not enough data is available a partial buffer
is returned.
Remove unnecessary value check

Due to the way the `render` function was rewritten in eabf971, the audio
variable is always going to be an array of `AudioSample`s, so a check to
see if it is equal to `nothing` is no longer applicable.
Add audio file seek capabilities
Make audio file reads return data as n_frames x n_channels
deprecates af_open in favor of AudioIO.open
some minor test file restructuring
adds a failing test for file playback
merges in @bauglir's changes
test for mono file playback now passing
now transposing the audio so columns are channels on a file read
now we have a failing test for stereo file playback
adds stereo wav test file
fixes issue and now passes test for stereo file playback (mixing to mono)