Feature/generate named constructor from beConstructedThrough

I feel like it would be a choice between having a public constructor OR a private constructor with named constructor methods. 

If you were going down the route of having named constructors, you'd definitely like to have a private constructor to ensure your object can only be created in the way you intended. 

And if you changed your design down the road to have a public constructor, you'd most likely remove your named constructor method(s) and private constructor from your class, and then phpspec could generate your new public constructor with beConstructedWith()