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)
```
Fix. Implement `skt:has_event` in ffi version.
Update test for `proxy_steerable` (ZMQ 3.0.6 still broken)
Update doc [ci skip]
Merge pull request #21 from moteus/master

Add. `skt:has_event()` method