New-MofFile updates

Fixed code around enums; underlying type needed to be translated to equivalent CIM type instead of putting the .NET framework type into the schema file.

Also revised code to check equality of type literals instead of doing string comparisons, which will make the code more robust when dealing with unknown type accelerators, System namespace included or excluded, etc.

This is a follow-up to the discussion in #71 .

There aren't any Pester tests for New-MofFile yet; we should add some soon.  In the short term, I tested these changes manually to ensure that good schema.mof files were being produced for known modules, as well as for properties with enumerated types as parameters.