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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While we couldn't do this until Spring Batch 3.1, I'm wondering if we actually need a new interface for this.  It's the same thing as the `LineCallbackHandler` that we already have, just that the existing interface is typed to a `String`.