Users can make orders to addresses in non-shipping countries

In the address book form countries are not filtered by *is_shipping_country=True*.
And this is ok: for example the user has a billing address not in a shipping country.

But you can set these addresses as default shipping address and they appear as valid shipping addresses. 
So users could make orders to addresses in non-shipping countries.

My suggestions are:
* The form should complain if a user submits an address with a NON shipping country and the checkbox 'set as default shipping address' checked.
* Addresses with a non-shipping country should not appear in the checkout process ([here](https://github.com/tangentlabs/django-oscar/blob/master/oscar/apps/checkout/views.py#L145))
* Country should appear in the shipping address page during the checkout even if there is only one shipping country to avoid misunderstandings.