Added links to some Pratt parser articles.
Move the token classes to the top because we're going to need
the class names defined earlier ini an upcoming commit.
...
Use "token" -> Class mappings instead of instantiating classes
with self.  This will eentually let the parser be moved into its own class
with the Token classes defined separately.
Slightly simpler DigitTokenFactory that just returns a new Object
instead of a new Class, because all it needs is a #new method.
Token instances are no longer constructed with the Evaluator.  Instead it
is passed to nud and led as needed.  Mostly this allows Evaluators
to be constructed without constructing a new set of Tokens.