Allow view create Response object

[xen]

Allow view to create custom Response object.

[xen]

Another approach can be support for second value in return with code value. Like this:

@blp.response(code=204):
def delete(self, pet_id):
    pet = Pet.get_by_id(pet_id)
    if pet is None:
        return None, 404
    Pet.delete(pet_id)

But one that I've implemented gives more flexibility.

[lafrech]

Thanks. I intend to do something like this.

I'd like to address #18 at the same time.

Basically, allow Blueprint.response to receive either a response and headers, or just a response, and the response could be just a payload or a Response object.

I just need to get my head around this.

[lafrech]

@xen, I just sent another PR implementing this (#40). Feedback welcome.