Added more Tests, removed more code from Token.
- EmptyTokenException --> TokenException
- TokenException constructor now takes a message String, and passes it
to super; this allows more flexibility and lets us pass better error
info.
- the 'isNumber' parts of Token were removed, and those functions were
replaced in other classesas well. 
- Added tests to cover a few more exception conditions, and
InfixToPostfix's string cleaning and Token list cleaning code.
- Removed a few bits of code from PostfixEvaluator; since it never gets
a minus token, we don't need to handle them.