Check for openpyxl
I added a try/except catch for the lack of an installed openpyxl. The user gets a nice message that she needs to install it or to save the .xlsx file as .xls.
I also added a test for it and a test.xlsx file that I saved from test.xls.
Furthermore I noticed an ugly break of the DRY principle in ExcelFile.parse and gave an alternative with a nice dictionary that provides the handle for the required method depending on the 'use_xlsx' flag.
