libavfilter: Add asettb filter for setting timebase for audio

Ported asettb (including the sr option for audio sample rate) from FFmpeg,
and copied/modified the existing settb documentation for asettb.

Signed-off-by: Katerina Barone-Adesi <katerinab+libav@gmail.com>
Signed-off-by: Anton Khirnov <anton@khirnov.net>