Change get_buffer call protocol

To make get_buffer() calls more natural in Python, make it return the
buffer or None if replay has ended. This works well with pyaudio.

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>
Address pylint warnings in the callback example

Signed-off-by: Claudio Matsuoka <cmatsuoka@gmail.com>