A new feature for dullard that can handle empty cells

Hi,
Thanks for this awesome super fast library that handles XLSX better than all other gems. I have been using dullard for the past few weeks and I noticed that the "rows" enumerator does not return empty cells. So I forked the code and created a new enumerator called 'full_rows' that includes empty cells in the returned rows.

I have written rspec unit tests  and fixtures for this new function and included it in the commit. So, Please feel free to merge this commit to your code base if you like this new functionality.

If you have any question, please do not hesitate to contact me.

Thanks,
-ramtin