A to-do list for the Gviz package

General structure:
   o introduce file-reference classes that draw genomic coordinates and associated data from bigBed, bigWig, BAM or tabix files

AnnotationTrack/GeneRegionTrack
   o get the data from TxDb objects
   o speed up .prepareGene (may need a C code module)
   o deal with additional annotation features (UTRs, etc.)
