DISCO: import UCSC known genes

UCSC keeps a list of known genes that coincides with their genome builds. we should take it in, and build a set similar to NCBI gene.

see: http://genome.ucsc.edu/FAQ/FAQdownloads.html#download31

alternatively, these could be fetched from their mysql public instance rather than ftp if desired. table is "knownGene". they also have mappings to other resources, like ensembl.