Fix #98: Implement download() method
Add CURLOPT_RETURNTRANSFER reset when using download method; Fix download method test failing on PHP 5.3 due to different png files being created on the server and client when using imagepng().
Fix "Operation not permitted" (PHP 5.3) and "Permission denied" (HHVM) errors
Merge pull request #101 from zachborboa/master

Fix #98: Implement download() method