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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I may be able to figure this out from the code below, but what defines a "row"?  With Excel, you can have a cell that spans multiple rows with merging so does that cell get included in both rows?