Support for target functions with multiple parameters, simple IEnumerable without property mapping, better error handling

Hi Govert
I checked in some changes to MapArrayFunctionRegistration over the weekend, which gives much better support for different function signatures.
The column header / property mapping is now turned off by default, you enable it with a parameter level attribute.
It now supports simple sequences, e.g. IEnumerable<int> automatically mapping to/from a single column Excel array.
And it supports functions with multiple parameters... pretty important.
Still more to do, but it's quite a nice way of mapping idiomatic C# and F# functions to Excel, without converting to object[,] yourself.
Let me know what you think!
Ben