basket.Line.quantity is a positive integer field, but the quantity sumation in
basket.Basket.add_product made it possible to try and save negative values. This
makes add_product never go below 0-quantity, thereby preventing a db exception
from being thrown. Also adds a test for the described behavior.

This solves an issue where a customer's voucher is dropped when they
sign in.

Fixes #1187

The word "stock info" was bugging me as it's not quite correct for the
combination of availability and price information.  The problem is the
word "stock" which implies physical widgets in a warehouse somewhere.
It sounds weird for ask for stock info for a digital product.

Hence, I came up with PurchaseInfo, which is a bit more vague but
captures the information better.

We no longer need to pass the StockInfo instance as the basket has a
reference to the strategy class and can fetch it for itself.  This means
we don't have to break backwards compatibility with Oscar < 0.6.

Plus provide a currency property on the basket itself.

- Restructure into unit and integration tests.
- Add more tests for the various basket properties.