HLS audio synchronization

Sometimes when playing HLS on iPhone you can hear crackles. The reason for them is MPEG-TS timestamps have much higher resolution than RTMP timestamps. Nginx-rtmp has a built-in feature fixing that. The directive name is hls_sync. I have not announced this feature before. Now I have added it to directives page. Moreover the feature is on by default. So if you have crackles in HLS just update from master.