Add an AbstractParser class

The deprecated parse() method in the Parser interface causes quite a few repetitive method declarations in all parser classes, so to simplify things I'd like to introduce a BaseParser class that provides reasonable default implementations of all Parser methods and can be used as the base class of any Parser implementation. It would be analogous to the DefaultHandler class in SAX, and would also make it easier for us to introduce new Parser methods if needed later on without necessarily breaking too many existing Parser classes.