Bugfixes, sheet size metadata support, dropping Ruby 1.9 support

This release fixes an issue where worksheets are accessed with the wrong filename, and fixes reading empty cells.

If worksheets have metadata for the number of rows in the sheet, it will be returned with rows.size. This feature breaks compatibility with Ruby < 2.0, but if there is user demand, I will try to backport the library to 1.9.

Thanks to @c0, @chewi, and @pythonicrubyist for contributing!