API: Creating an Order as guest (#2545)

Hi,

   As I mentioned before, it would be very nice to be able to create an Order, through the API, as a guest, just like if it was being created by a customer navigating in the store-front. 

   So, here follows my proposal to do this. It is important to notice that we should be able to create Users and assign them to Orders, also, to be able to implement "logged" checkouts.

  What do you think?


Regards!
Tiago.