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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The issue with a `RowMapper` concept is that you are missing the `ResultSet` piece of it.  If you're going to use this approach, you'll need some form of wrapper that represents a combination of the meta-data and the data.  In a JDBC `RowMapper`, it's the `ResultSet`.  In the `FlatFileItemReader`, it's the `FieldSet`.  In your case, you need some form of result set like representation of the meta-data and data to pass to the method instead of passing the `Sheet` around everywhere.