Added 4 new Excel Spreadsheet Exporters for testing

+ The "meat" of this code came from this commit [ https://github.com/harvard-library/Vireo/commit/f968e58c9606cfe70416f9becb04cea001071ae8 ] from this pull request [ https://github.com/TexasDigitalLibrary/Vireo/pull/48 ] but was modified to implement/integrate it into the Export Job framework within Vireo.
+ ExcelExport - Just like FileExport(Zip file containing one directory for each submission) but with an added xlsx data file in each submission's folder (all columns/metadata)
+ ExcelExportColumnFiltered - Just like ExcelExport but it only exports to the xlsx the columns/metadata visible in the submissions List in the UI
+ ExcelExportAggregate - A single xlsx file containing all of the submissions in the submissions List in the UI (all columns/metadata)
+ ExcelExportAggregateColumnFiltered - Just like ExcelExportAggregate but it only exports to the xlsx the columns/metadata visible in the submissions List in the UI