Fixed problem when processing candidates where next_item only 
went to the next source, not the next unprocessed source.  This meant 
when reloading from partial progress in a file you would only get bumped 
up to the second source, and if you had really already processed that one
it wouldn't get downloaded again.  Therefore you were left with an image
loading dialog forever.
Merge pull request #81 from drusk/cands_load_ff

Loading partial progress for candidates now fast-forwards