BATCH-2041: Add support for reading excel files

This commit adds support for reading Excel files. Both JExcel and Apache POI are supported. Support for reading Excel files has been designed similar to the support for reading flat files.

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Actually I'm not sure. Removing JExcel would allow for a lot of simplifications as the abstraction could largely be removed. It only exists to accommodate both implementations if we drop one it would be a lot easier.

Making it deprecated and removing it in a next versions doesn't make much sense either I guess.