Add CURL support

Some shared hosting does not allow socket connection. With native curl library support, people (like me in my shared hosting) may have choices to use curl as its connection library.

P.S.: How to build the documentation? I am sorry, i don't know how to build the doc. Once i know how to build it, i will also update the doc.
Thanks. [In curl_exec documentation](http://php.net/manual/en/function.curl-exec.php), it should be an error if it returns `FALSE`. Will be fixed on next commit.