Several enhancements to the Excel Type Provider

1) Upgrading to the latest version of the ExcelDataReader library.
2) Handling loading higher version of ICSharpCode.SharpZipLib.
3) Defaulting to the first sheet in the spreadsheet if a sheet is not specifically indicated.
4) Using a Row type, with a ToString() method

These fixes will make the F# Excel Type Provider more usable, and help address some of the issues causing https://github.com/fsprojects/fsharpx/issues/274. To completely fix this issue, we will need to publish an updated NuGet package.

I would be glad to update the configuration for the NuGet package if someone could offer some guidance as to where the existing configuration is.