v2.0.1

Add parameter heartbeatTimeout to the constructor of each class

heartbeatTimeout is timeout(milliseconds) for waiting heartbeat message, default 500ms

1. A heartbeat message will be sent to peer immediately when a request message is received
   and there is at least one handler for it. Or the emit method will catch a no handler error
   It has nothing to do with the time of handler execution, there is no timeout
   for handler execution
2. Normally, a heartbeat message will be sent to peer in less then 10 ms,
   but you may still need to set a longer timeout if browser is heavy loaded
   and the native apis are slow