Allow for pickling/deepcopy of wordnet objects

[goodmami]

Context: nltk/wordnet#28

Currently objects like Synset cannot be deepcopied or pickled because they store a connection to the database. Removing that allows them to be copied/pickled, but these objects still contain database rowids which means they cannot be stored and loaded on some other build of the database (e.g., another machine). Either we stop storing the rowids (hopefully without too big a performance hit) or we write a custom deepcopy method.