Refactor some classes to improve readability and maintainability

[ntung]

I used to look into the source code and intended to change some areas to make the codes more readable and maintainable. Is that worth?

For example:
When looking into the PR #282, I read the private method readXMLFromXMLEventReader spanning over 300 lines. It's actually hard to read and understand the logic implementation.

I can point out some other non-standards: code lines too long, some typos in Java docs, poor function docs, unused function arguments, a lot of uncomment codes, duplicated codes, redefined logger inside a method, etc.

If we agree this suggestion, I can contribute my time to work on these things gradually.