Add support for excel spreadsheets to application import.

[jortel]

Support Excel spreadsheet (format) in addition to CSV.

[aufi]

I'm happy to look on this task, I might have few question to clarify:

• do we want support xls and/or xlsx file formats?
• should order or columns in the spreadsheet matter? (like Tackle 2 csv import) or their title should matter
• is the expected table format be the same as in csv import?