SetStreamHandler(protocolID, handler) and NewStream(peerID, protocolID) bindings

[moul]

• This will allow developers using gomobile-ipfs to simply establish p2p connections and implement logic around them.
• Example: a developer wants to create an application that displays notifications from a smartphone on a desktop PC without using servers will be able to use gomobile-ipfs to simply establish a direct connection between the two devices and implement the notifications transfer logic in the handlers.
• TODO: add code example for SetStreamHandler(protocolID, handler) and NewStream(peerID, protocolID) usage