Lowest Common Hypernyms Change & Tests

- Set lowest_common_hypernyms to use max_depth by default
- Updated wup_similarity() to use min_depth with LCH
- Added a tests file for wordnet (only tests lowest_common_hypernyms()
at the moment)
- Added an additional LCH example to demo() which illustrates a case
where a target synset is also the LCH for itself and another synset