corpkit plus ReadTheDocs

The main thing going on now is some decent docstrings, which allow for some decent documentation via http://corpkit.readthedocs.org/en/latest/. Since the last release, things have also gotten more stable. Corpus class, and its subclasses, are working really nicely: it's now easy to search particular subcorpora, multiprocess, or treat files as subcorpora. the interrogate method has also impoved a lot. conc has been subsumed within interrogate. All is well.