XLSExport: A CLR procedure to export proc results to Excel

Introduction

NPOI is a .NET project which can read and write read/write xls, doc, and ppt files without the need for a Microsoft Office installation or license. Formerly on CodePlex, the project is now located on Googlecode at http://code.google.com/p/npoi/.
In this CLR example, I'll use NPOI to create a procedure that takes a stored procedure name and list of parameters and parameter values for that procedure, and outputs an Excel spreadsheet that contains the execution results of that proc to the passed .XLS file specification. The advantage to a CLR solution is that (in addition to lacking an MS Office install or license requirement) ActiveX and COM components are not required on your otherwise pristine SQL Server to accomplish this task.
This procedure supports output data types of CHAR, NCHAR, VARCHAR, NVARCHAR, all numerics, DATE, TIME, DATETIME, DATETIME2, BIT, and BOOLEAN. Other datatype outputs are not currently supported, but are