Made discount codes with unset uses remaining work.

SQL generated for ``exclude(uses_remaining=0)`` is ``where not (uses_remaining=0)``, and this will filter out any null values too.
http://en.wikipedia.org/wiki/SQL#Null_and_three-valued_logic_.283VL.29
Merge branch 'bugfix/discount-codes-without-uses' into feature/loyalty-discounts

Conflicts:
	cartridge/shop/managers.py
Merge branch 'feature/loyalty-discounts' into develop