Enable specification of external assemblies for controller discovery …

...ServiceStack.Mvc.FunqControllerFactory.  Unit tests added to preserve default behaviour in:

tests\ServiceStack.Mvc.Tests\
tests\ServiceStack.Mvc.Stubs.Tests\

New projects added to base solution

src\ServiceStack.sln

Happy to update other solutions if necessary - and any build related fixes this may impact.

Related issue : https://github.com/ServiceStack/ServiceStack/issues/599