Merge pull request #9 from zhaozg/master

Fix. Build with libcurl 7.19.7 (Close #8)
Fix. Handle CURLM_CALL_MULTI_PERFORM to support libcurl < 7.20.0
Fix. `version_info` returns protocol names in upper case.
Change. `info_read` could automatically remove easy handle if it done.
Fix. Windows detection.
Add. Export error classes (ERROR_EASY, ERROR_MULTI)

```Lua
e = curl.error(curl.ERROR_EASY, curl.E_UNKNOWN_OPTION)
```
Add. `category` method to error object.
Update. Object wrapper int examples.