adding thumbnail support to s3 uploaded images

I modified the Thumbnail.php file to utilize the S3 utility. It was failing on file_exists() checks since the files aren't within the file system. In general I modified it so that anywhere it anticipated a file in the system it would instead use  a "tmp" file using a PHP image resource.

I added another config to S3 in the main config.php file for 'baseURL'. I put it here instead of using the global $baseDir because I wanted to allow for someone with a custom cname to use a different base URL for the files than they use for connecting.