Enable NCrunch.
Create an instance of the requested type.

We get quite a lot of code out of this first test, because where
previously there was nothing, there is now something.
Add type restriction to `BuilderFor<T>()`.

This library is only going to be useful for classes, so restrict the
factory method to only class types.
The factory method must return `dynamic`.

This is to ensure the dynamic dispatch functionality is enabled.