Fixed an incorrect mkdir_p call with correct path before creating the zip file. Normally on non-Windows platform, this does not cause trouble (just extra sub-directories created). But on Windows, since the path would contain drive like 'C:' which would failed the call to mkdir_p.
Merge pull request #56 from bryantsai/workbook_write_incorrect_mkdir

Fixed an incorrect mkdir_p call with correct path before creating the zi...