* need to document --sizes option

* make iter(Chromosome) and friends return supercontigs in order
* add supercontig gap length variable
* add sequence length variable (set during load_seq)
** allow full slicing once sequence length known for each chromosome

* change supercontig.start and supercontig.end to unsigned ints, not
  signed ints. then compile with -Wextra

* needed tests:
  ** multiple track headers in middle of file
  ** tab-delimited track header

* run coverage
  <http://www.nedbatchelder.com/code/modules/coverage.html>

* run fuzz

* buildbot

* C API
