Fix parsing of .xlsx files exported from Numbers

Numbers 3.1 can export .xlsx files, but the feature has some
incensistencies and bugs.

This commit fixes an inconsistency where the first sheet doesn't have
have an index in the XML filename ("sheet.xml"), whereas such sheet
would be named "sheet1.xml" by Excel.

This fix is inspired by a solution provided in #110.
Merge pull request #114 from sidonath/fix-numbers-xlsx-parsing

Fix parsing of .xlsx files exported from Numbers