Implements constructors as interfaces

As mentioned in #1 I've got an error on property `super_` when asserting my inherited constructor in nodejs. The problem was not to ignore this property. The problem was that I expected that constructors are checked by its prototype methods. But it wasn't, so I implemented it.

With this pull request, any function passed as _expected_ will be instanciated befor checking. This can be disabled by setting the option `instanciateConstructors` to `false`.