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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Initially it was for a String[] (the first representation of a `Row`) and I didn't want to depend on the reading mechanism for flat files. But if this is changed in 3.1 (made generic probably) we could remove it by then.