added a random baseline for signified-only feature handler, see https://www.pivotaltracker.com/story/show/51881195 . To achieve this refactored  bov to access the thesaurus at decode time only through a callable (self.thesaurus_getter). This callable is passed on to the respective feature handler. Also added a unit test for this functionality and a few minor tweaks to enable to program to work without a thesaurus in the conf file.
1) Changed theseurus_loader to never return None, and bov vectorizer to never revert to super behaviour in _term_count_dicts_to_matrix. This ensures all vectorization goes through the same token handlers, which can log stuff. This will not make any difference to experiments that read a thesaurus from disk.
2) Minor tweaks: gave a few things more meaningful names, removed unnecessary code, etc