Add filtering by user agent string

[GoogleCodeExporter]

I'd like an application to use the script to resize artwork from anywhere 
(ALLOW_ALL_EXTERNAL_SITES = TRUE). In order to "harden" it I'd like the script 
to check for a specific user agent string.

The attached patch should add this simple check. 

Original issue reported on code.google.com by michael....@gmail.com on 16 Sep 2014 at 2:02

Attachments:

• ua-check.diff