Empty nodes not included in brat visualization

[spyysalo]

Reported by @fginter

[spyysalo]

Related to #15. The issue is ConllU.Document.toBrat()

[spyysalo]

4e38808 introduces a candidate fix by adding an includeEmpty argument to ConllU.Document.toBrat(). (The commit also brings in related stuff previously committed to the gh-pages branch that had failed to make its way into master.)