The local PeerConnectionWrapper should store the data channel returned by RTCPeerConnection.createDataChannel().
Issue-Id: #12
Issue-Id: #13
New method to pass received messages out of PeerConnectionWrapper
Split SinglePageChatTest into two suites: before and after connection is established
SinglePageChatTest: add empty line before each test
SinglePageChatTest: add suite setup methods
DataChannelSpy: add onmessage event handler
PeerConnectionWrapperTest: test messages with fake channel