Merge pull request #4 from zhaozg/master

fix warning on msvc
Merge pull request #7 from zhaozg/master

fix mem leaks bug, unref cb data when easy/multi cleanup
Fix. Build with old version libcurl (7.19.7)
Fix. Build with old version libcurl (7.19.7) (2)
Fix. Build with old version libcurl (7.19.7) (3)
according to symbols-in-versions in curl.