code: def sequence(db, chrom, start, end):
      
      url = "http://genome.ucsc.edu/cgi-bin/das/%s" % db
      url += "/dna?segment=%s:%i,%i"
      xml = U.urlopen(url % (chrom, start, end)).read()
      return _seq_from_xml(xml)
docs: return the sequence for a region using the UCSC DAS
      server. note the start is 1-based
      each feature will have it's own .sequence method which sends
      the correct start and end to this function.
  
      >>> sequence('hg18', 'chr2', 2223, 2230)
      'caacttag'