Split off ResponseApi from Client.
Handling the Request-Response pattern has been moved to a dedicated
ResponseApi class. This helps keeping concerns separated and prepares
this project to support the PubSub pattern.
Register event handlers only when the API is busy
Rename Client::send() to sendRequest() and add sendMessage()
Update readme example to use new ReponseApi
Rename ReponseApi to RequestApi :)
Merge pull request #12 from clue/responseapi

Split off ResponseApi from Client