Allow get_availability to accept quantity parameter.

In some cases where a product must be sold in multiples (extensions such as Bundles may enforce a certain/min quantity on a product, for example), it might help to have availability information taking a min quantity into account.

For example, a product sold in a bundle in fixed increments of 2 should appear as out of stock / on backorder *inside the bundle* if there is only 1 item left. In these cases, ```get_availability``` can be re-used by adding a quantity parameter, without having to filter it, or simply re-write it from scratch for something so simple.