Support custom JSON Message in UIKit

[chpapa]

1. Support when the UIKit received a special JSON message, the UIKit will call a callback function for a custom view to be embedded in the view.

2. Support adding a custom icon in send message area, when clicked, it call a callback function for a custom view to embed in the keyboard area of send message, and user can send a message with a custom JSON meta data.

Android relevant issue: SkygearIO/chat-SDK-Android#90