Install-IISAppPool - need to be able to set 'No Managed Code'

Install-IISAppPool currently uses ValidateSet to ensure .NET CLR is 1.0, 2.0, etc. But Dot Net Core 1.0 needs an AppPool with Managed Code Disabled. See here

Get-IISAppPool returns an empty string for 'No Managed Code' so presumably Install-IISAppPool needs to accept the same. Or maybe have both return/accept 'None' since an empty string is a bit... odd.

Fix issue #210: Install-IisAppPool can't set .NET framework version to 'No Managed Code'.

→ <<cset ad81d271972e>>