Update the code to support iOS6 and above

1. Replace deprecated Audio Session Services with iOS7 friendly AVAudioSession methods.
2. Change the way of handling audio route change event, especially for bluetooth device switching between available and unavailable.
3. Remove the two notifications. Instead of post notification, if someone wants to handle DevicesAvailableChange or AudioDeviceChange, he should use RACObserve(availableAudioDevices or audioDevice).