Added new test for TabbedReport

fixed: self.filter_field must point a real field name

tabbedreport test will add another report, and a tabbed report to
contain both reports. Fetch the xls, and verify that two correctly named
sheets exist in the workbook

fixed: download_tabbed_xlsx - optimized_write=True does not create the
default sheet. encoding should probably be utf-8

refs #98
Applied pep8. refs #98