Add. easy/multi handle support user values.

```Lua
  f = io.open(...)
  e = curl.easy():setdata(f)
  ...
  -- when easy done
  e:getdata():close()
```
Fix. cURLv3 `info_read` return correct object.
Fix. socket callback get correct cURLv3 easy object.
Update doc.
Fix. multi iterator in v2 and v3 API.
Merge pull request #47 from moteus/master

Fix. cURL API may return lcurl raw handle