uv_poll bindings

The reason for which I added uv_poll is that I have a library which provides a file descriptor and a call for handling the fd. I'd rather not run that in a separate thread.

Now, I reckon it's easy to shoot yourself in the leg with this feature because if you leave the fd hanging after it's been written to, it will crash the loop; i.e. it must be read when the event comes. However, being a C programmer, I'd rather have that privilege than not.