Remove EventLoop::register, rename register_opt to register

[alexcrichton]

The register API probably doesn't have the right default for most use cases, and most implementations end up calling register_opt anyway. This should probably be reflected by removing register and renaming register_opt to take its place.

[carllerche]

I am in favor of this. I would even say that there is no good default within context of a single code base using mio.