Improve performance by not keeping all comment tokens in memory

I found that 1.0.9 has significant memory footprint than the google code version. The reason is that it keeps all comment tokens in memory.

This pull request tries to just keep the last token instead of all tokens. I'm new to javacc, but I think this is correct, as it's been verified against a few large codebase (e.g. openjdk, hadoop) on my box.