Set IgnoredAssemblies when initializing AppDomainAssemblyTypeScanner

The value of IgnoredAssemblies is null when  AppDomainAssemblyTypeScanner#UpdateAssemblies initializes, which causes problems when scanning certain assemblies (like Microsoft.Web.Mvc).

Another fix is to simply catch the type load exception (https://github.com/runesoerensen/Nancy/compare/fix-typeload-exception) when "GetExportedTypes" is loaded, but it seems like the above solution would allow for the (supposedly intended) configurability of what assemblies should be ignored.