Test refactor with IteratorTestCase
Moves assertions on IteratorTestCase
Adds support for all spreadsheet formats supported by phpoffice/phpexcel

Using phpoffice/phpexcel now we support csv, xls, xlsx and the other
formats supported by phpoffice/phpexcel.
The naming has been improved from AssociativeCsvIterator to,
more generic, AssociativeSpreadsheetIterator.
An XLSX specific test has been added.
Removes multiple values processing
Adds README.md