add audio defaults and limits calculation to libhb

add audio defaults and limits calculation to libhb

hb_get_audio_bitrate_limits()
	Get the bitrate limits for a (codec,samplerate,mixdown) triplet

hb_get_best_audio_bitrate()
	Given an input bitrate, sanitize it. Check low and high limits
	and make sure it is in the set of allowed bitrates.

hb_get_default_audio_bitrate()
	Get the default bitrate for a given (codec,samplerate,mixdown) triplet

git-svn-id: svn://localhost/HandBrake/trunk@3578 b64f7644-9d1e-0410-96f1-a4d463321fa5