Avoid 100% CPU on curl_multi_exec

Never finishes (up to max_execlution_limit) on PHP 5.4.24.

See http://nz1.php.net/curl_multi_exec#113002
and http://www.adrianworlddesign.com/Knowledge-Base/php/Download-content-with-cURL/Doing-curlmultiexec-the-right-way
Merge pull request #9 from silverstripe-labs/pulls/cpu-usage-curl-multi

Avoid 100% CPU on curl_multi_exec