Utility Functions for Abstraction layer of Audio / Video message attachment

[chpapa]

UIKit got audio / video sending ability: #141 SkygearIO/chat-SDK-Android#50 SkygearIO/chat-SDK-iOS#64

But a lot of these functionality should be platform specific non-UI related, so maybe we should have utility functions in the SDK for those who don't use UIKit

Illustration examples, not spec

Core API:

• sendMessageWithAudio(message, audiofile)

Utility Functions:

• initAudioRecording()
• recordingStart()
• recordingPause()
• recordingStop()
• setRecordFinishCallBack(func)
• playRecording()

Other thought:

• I felt like this case is similar to UserTypingIndicator in chat