Call StanfordLemmatizer explicitly

This is required for dkpro 1.7.0 compatibility, it seems that implicit
lemmatization by StanfordParser is going away.
TreeUtil.getAllGoverned(): Fix -of- traversal for dkpro 1.7.0
QuestionTypes.Clue: Add specific types for various clues

This change will allow (i) more flexible determination of which
clues are mandatory in solr search, and (ii) easier clue weighting
parametrization.
question.ClueBySubject: Make both ClueSubject use weight 2.5

We'd like to have all specific clue type instances to have the same
weight.
SolrTerm.required: New attribute allowing fine-tuning of required terms
SolrTerm.cluesToTerms(): Required only for ClueNE, ClueSV, ClueSubject