Add. `skt:has_event` method

Usage:
```Lua
-- Polling zmq socket using libuv
-- use lua-lluv binding

uv.poll_socket(sock:fd()):start(function(handle, err)
  while sock:has_event(zmq.POLLIN) do
    local msg, err = sock:recv(zmq.DONTWAIT)
    ...
  end
end)
```