Add. `skt:has_event()` method

Usage:
```Lua
-- Polling zmq socket using libuv (use lua-lluv binding)

-- start polling zmq socket (e.g. ZMQ_SUB)
uv.poll_socket(sock:fd()):start(function(handle, err)
  -- We need read all messages from zmq socket
  while sock:has_event(zmq.POLLIN) do
    local msg, err = sock:recv() -- we have no need use zmq.DONTWAIT
    ...
  end
end)
```