Misc. improvements to TruthTable and AllowedOperators.

- Made some changes to some method signatures to make them generic.
- Added getOperatorByString method to AllowedOperators.
- Bug Fix: TruthTable's iterator's hasNext method fixed.

Type: Bug Fix
Type: New Feature
Type: Code Improvement
Added the ParserTools class.

- Added the ParserTools class that provides several methods useful
  for string processing, and parsing a Boolean Function expression.

Type: New Feature