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.

I have signed and agree to the terms of the SpringSource Individual Contributor License Agreement.
Going back to the `AbstractExcelItemReader#close()` comment, this class should really override that method and call `this.workbook.close()` in which should close the `InputStream`.  Also, per the `WorkbookFactory` javadoc, the `InputStream` must support mark/reset to be used.  We probably should test for that support here before using it and throw an exception if it's not supported (see https://poi.apache.org/apidocs/org/apache/poi/ss/usermodel/WorkbookFactory.html#create(java.io.InputStream) )