Fix assembly hub bug with non-binary trees

Closes #7.
Merge branch 'development'
maf tree WIP
hal2maf block trees mostly working

known bugs:
- ancestral references don't appear at top
- doesn't work correctly with restricting to a set of genomes / a
  subtree
Merge branch 'development' of https://github.com/glennhickey/hal
Merge branch 'development' of https://github.com/glennhickey/hal into development
Merge branch 'master' into development