Add Stripe API Key settings to close #11

Removes the hardcoded API key and replaces it with a new Settings
page in the backend. Also adds a setting for the active keyset,
in case anyone wants to use test mode while in production.
Autoset qty if not defined by user, closes #3

If you try passing a null qty to the Cart class, it bails. We get
round that simply by falling back to a qty of 1 if it's not set
in the post vars.
Remove basket items that don't exist, closes #5

There's always the chance someone could add something to their basket,
only for the store owner to delete it before they get to checkout.

Rather than having non-existant items added to orders, this code will
remove any missing products from the basket and go back to the checkout
page so they have chance to decide whether they still want to order.
Merge branch 'feature/quick-tweaks' into develop