Refactor Client Code

 - Implement JSONRPCClient class
 - Implement RegistryClient (Concrete JSONRPCClient)
   - Serialize Ticket objects
 - Refactor ZMQTicketRegistry
   - Don't do JSON/JSON-RPC/Data-Serialisation
 - Fix some bugs in JSONRPCServer
   - 'id' does not have to be a string object