iOS 8 fixes, dynamic sample rate

- Sample rate is not “forced” to the hardware anymore, but the
hardware’s native sample rate is accepted.
- You can set the preferred minimum sample rate to always have good
sound quality.
- The preferred buffer size is set by milliseconds now, as the sample
rate may change dynamically.
- iOS 8 has multiroute audio session bugs with several USB audio
devices (silent output). Now the object turns on the multiroute audio
session category for 2-channels USB/HDMI devices only. Multichannel
audio devices are handled with other audio session categories (no
multiroute session necessary).