Added Excel XSSF parser with single row callback

Hello, 

My team came across your Excel Reader for XSSF SAX parsing while refactoring some code to be more memory efficient on reading excel files. This pull request includes an Excel handler inspired by yours, but customized to allow a developer to perform per-row processing and only having one row of data in memory at a time.

I think this would make a nice addition to your Excel Reader utilities.

Thanks!