fix deprecated BlobBuilder 

I've just load your extension today and found the save function to be broken somehow. So, I dig down a bit and found the cause of problem. "WebKitBlobBuilder" is undefined and deprecated (according to http://updates.html5rocks.com/2012/06/Don-t-Build-Blobs-Construct-Them). So, here's a quick fix from me. Enjoy! :D
ps my chrome version is 24.0.1312.57m in Windows 7