Overhauls the class definition. Merges Corpus class with
LexVar class. This makes it possible to calculate all the
Celex variables using the module celex.py.

Adds lemma_headwords method to find the headword
of a word key. Adds optional argument use_lemma to
functions that require the lemma headword instead of
the word. This is not truly optional because the
function will break otherwise--more of a note.