Capture nil for relation in TreebankRetriever

... and blank it, so as not to distort the unused counts.
Add .bold css class
Allow to specify a empty-val for valueWatch

... used in dependencyTree to highlight unset values better.
Add bold class for empty valueWatch values
Merge pull request #347 from latin-language-toolkit/relation_nils

Updates to valueWatch (relation label styling) and relation label parsing