Implement puzzle score function

[timpalpant]

This should be a callable that takes a Crossword and returns a float.

Scale can be arbitrary, lower scores are better. There are some ideas in the Dr. Fill paper for components that are informative. To start, we want something that:

1. Assigns lowest score to things in the ClueDB,
2. Assigns lower scores to real words/phrases,
3. Assigns lower scores to more probable sequences of N-grams

It can have parameters to weight these components, and maybe a script to fit the parameters.

This will be useful for 1) selecting the best solutions generated by the solvers and/or 2) directing the search.