ensure minimum set of header column names for choices and survey
exist. Displays a clear error message.
updated settings.xls and test to reflect current specs: using label column
ensure xls_dict and csv_dict generate same dict
updated fixture to march specs and ensure expected output is generated
check that we have column headers for choices and survey sheets
ignore libre office temp files
return correct error message if choices sheet is missing or badly named, resolves #18
Merge pull request #85 from modilabs/missing_headers

better error messages when headers are missing
closes #17
closes #18