Documentation

This PR contains the documentation I've been working on at https://github.com/libuv/api-docs

In order to avoid comment rot (which already happened) I cleaned up uv.h. The API documentation already contains everything uv.h had plus more.

I don't expect anyone to go and prof read it :-) It's not perfect but all API calls are documented, unless I inadvertently missed something. So, trust me on this one and lets keep improving these as we go along.