Let Parser class return integers for certain tokens
Remove useless line
Update copyright information
Add exception classes
Add test cases for input data validation
Implement imput data validation
Do not document helper method
Use nil instead of empty string as default comment
Improve handling of comment strings
Add missing test cases and fix array validation
Extract method
Do not rely on specific format of exception message

Rubinius does not include the value in the exception message.
Small refactorings
Update Ruby versions in README