Force cfunc in queue_work's work_cb and add test for it.
Merge pull request #32 from take-cheeze/uv_queue_work

Force cfunc in queue_work's work_cb and add test for it.
Implement class UV::Req .
Protect allocated UV::Req from GC.
Use UV::Req in UV.getaddrinfo .
Release used getaddrinfo req.
Add `block` field to mrb_uv_req_t .
Use mrb_uv_req_t in UV.queue_work .
Fix and add test for UV::Req#type .
Undef UV::Req.new .