testing fix: nltk.clean_html was deprecated/removed
nltk.sem.chat80: raise ImportError exception when sqlite3 is not available.

This code previously raised NameError in such cases.
testing fix: don't import from deprecated and removed sklearn.svm.sparse
framenet.py: link together objects rather than storing pointers
framenet.py: Python 3 compatibility
Merge pull request #482 from nschneid/patch-6

framenet.py: link together objects rather than storing pointers; new propagate_semtypes() method
dropped duplicate content about framenet