support organism icons

[kzuberi]

The GeneMANIA website will include icons representing each of the available organisms. These are currently configured in the website itself, and available at bio-icons. It was suggested to make this configurable in the organism data itself.

This would involve updating data/organism.cfg to include a field for the icon filename, propagating this through to result/generic_db/ORGANISMS.txt and updating the lucene index to include this field. We might also need to update the data packaging if we want to be able to bundle the icon file itself with the resulting dataset.