Implement UV::Req.

[take-cheeze]

UV::Req is a binding of uv_req_t.

It would be returned when subclass of uv_req_t is instantiated to create request in libuv.
Only UV.getaddrinfo and UV.queue_work supports it currently.

It has following methods:

• type: returns uv_req_t type symbol mapped with UV_REQ_TYPE_MAP
• cancel: binding of uv_cancel. Not available for windows since libuv doesn't support it.

UV::Req.new is undefined since mruby script shouldn't create it directly.

[mattn]

UV::Req is is what I was thinking that I must implements. Thank you.