Implemented hasModifierSynset() @ MITWordnetUtils

- Implemented hasModifierSynset(), checking if a synset has the
modifier in its hypernym tree.
- listAllSynsets() at MITWordnetUtils has been changed to ignore
Instances of “leaf” synsets, since they simply implement other sunsets
already returned by the method.
- Changed return of listAllSynsets() to a map of HashSets instead of a
map of ArrayLists.