Refactor, optimize, clean-up.

The two games have a lot in common, so factored out a base class called
WordChain that implements most of the logic.  The game-specific behaviors
are implemented in the (now smaller) WordTwist and WordShrink classes.

Optimized to build a map of word relationships first and then derive all game
behavior from that map.  This could probably be improved even more, but for
the moment it is working nicely.
Improve WordShrink's is_trivial definition.

No substrings should occur in any word of any solution (not just two
successive words).  This is really going to narrow down the set of puzzles,
hopefully it doesn't churn or get repetitive.
Update copyright information.
Revise game difficulty settings.

Instead of taking lengths, the games now take easy|medium|hard|evil.
These values correspond to a range of puzzle lengths, word lengths, and
number of possible solutions.  I attempted to tune them to reasonable
values, but I could see them changing.

Also did a little more code clean-up.
Fix a performance issue in _find_solutions.

WordTwist successors can take you in circles (scare > stare > scare),
so check for this condition to avoid generating overly many potential
solutions.
Filter duplicate words in puzzles.

Was occasionally generating a puzzle foo > --- > --- > foo.  Oops.
Oops, handle missing word file gracefully.

Apparently I backed out the previous change that handled this.
Minor wording fix.
Change default dictionary and improve configurability.

Now defaults to the /usr/share/dict/american-english dictionary which is
probably found on many Linux systems today (avoiding the need to dig up a word
file on the interwebs).  On Debian/Ubuntu, you can 'apt-get install wamerican'.

Added a configurable regexp to filter the word list down to reasonable words.
Defaults to allow lowercase a-z only, therefore filtering out proper names,
hyphenations, contractions, and words with accented characters like "adiós".
(But hopefully still supporting non-English users by allowing this to be
changed.)
Update the command help in README.
More README updates.
Remove some useless checks.

Some solution length logic was previously moved to handle_message.  Some of
the code in _valid_solution became nonsensical (not broken, just weird).

Cleaned up.
Make _start_game arguments generic.

It shouldn't need to know what specific parameters the game will take.
(And its "length" parameter was out of date since the games currently
use "difficulty".)
Add a new game: Worddle

This is a clone of a famous game involving a 4x4 grid of random letters.
It uses Plugin's inFilter to filter out private messages to the bot during
the game, so that they are not treated as commands.  You can still send
commands to the bot using the command character during this period.  When
the game ends, the filter puts things back to normal.

Also implemented a new command when DEBUG is True, wordsolve, which shows
the solution to the current wordgame.
General improvements to Worddle gameplay.

Added support for broadcasting messages in one shot if the IRC server
supports multiple targets. (Inspircd only right now, need to add support for
more servers.)  This will hopefully reduce flood issues and latency.

Cleaned up messages, improved use of color, added join notifications,
added a 'get ready' state 5 seconds before game starts.

Some code cleanup and refactoring.
Add technical setup note to README.
Turn debug mode off.

Did't mean to leave this on.
Fix regression in WordChain games.

Somehow I took out the code that handles public messages during a game.
Only allow join if a game is running.
Factor out Worddle results into class.

Also, results are now presented in descending order by score.