Remove locking code and make it possible to have a dummy history, by calling the parameterless constructor.
* BrainFrontend.cs: Since History is no longer thread safe, lock
  around it. Also copy array to avoid messing up the input to
  BuildResponse.

* Database.cs: Improve WordCount handling. Also some other minor
  changes.

* MarkovTools.cs: Minor changes.
* BrainFrontend.cs: Add a comment.

* History.cs: Allow the constructor to be called with length of less
  than 1.
Small changes and a version bump.
Add a TrimPunctuation extension method.
Make Normalize also trim leading and trailing punctuation.