LibUV: Implement LibuvEventPoll

LibUV event will use the best available polling method on a system, kind of
like aria2 does already with the different *EventPoll implementations. 
However, libuv may support different/newer polling mechanisms; for example on
Windows it will use IO Completion Ports which are superior to select() ;)

This stuff isn't ready just yet.

First of all, building libuv with mingw-w64 sucks:
https://sourceforge.net/tracker/?func=detail&aid=3610440&group_id=202880&atid=983354
https://github.com/joyent/libuv/pull/774

And then, it might be good, or not, to not prefer libuv over all other
implementations. (Over win select() certainly :p)

Also, I didn't test-drive this enough.
So it's a WIP. Feedback welcome.