downloads a .2bit genome from UCSC .. note:: please comply with the restrictions of use at http://hgdownload.cse.ucsc.edu/downloads.html
and do not over-use this module
By default, genomes are saved to the current directory
tries to download a genome from UCSC by name
for example, ‘hg19’ is at ftp://hgdownload.cse.ucsc.edu/goldenPath/hg19/bigZips/hg19.2bit