Added lots of new game functionality.

Many different things have been added or modified, and shame on me
for not keeping up with my commits, so I will just cover the biggest.

1. The Wordlist needed for recognizing valid words has been added,
   the code being courtesy me and the source for the wordlist being
   courtesy of the English Open Word List at
   http://dreamsteep.com/projects/the-english-open-word-list.html.
   The game now recognizes valid words on the board as blocks come
   down but does not do anything with them yet.
   NOTE: The word list, by default, contains swear words and some
   unsavory language (mainly racial slurs) which were not added by
   me, and I do not intend on putting them into the final game.
   Eventually, I'll filter those out, but I just wanted to have
   that as a warning.

2. Letter generation is now based on letter frequency data for
   English found on Wikipedia.
   As is, the code has an added dependency on the Python Excel
   package at http://www.python-excel.org/ because I forgot about
   native csv support. I'll probably remove that later on, but for
   now, xlrd is pretty awesome.

3. I think that's it for the major stuff, but there is certainly
   a lot of minor stuff that has been changed.