Created an executable which only processes ITIS

Given the architecture of this software, I think it'd be useful to have a system so that only a single data source is downloaded and processed. The easiest way to do this would be to have a separate binary script which processes each type. I've created dwca-hunt-itis.rb; do you think that'd be helpful? It definitely makes life easier for my itis-dwca generating script! --> https://github.com/gaurav/itis-dwca/blob/master/code/download.pl