Add new tokens; Refactor and new impl. for tokens

- Add new tokens for ENTITY, NOTATION, ELEMENT and ATTRLIST declaration
- Add implemenatation for beforementioned tokens
- Change ErrorToken to collect processed text
- Changed visibility of self.read() method
- Added processing for get_entity_ref_token() and get_peref_token()
- Add implementation for '+' '*' '|'  tokens
- Comments now identify '--' characters as errors