Customize Magento Availability Message

On product pages, it says "Availability: In stock". We need to add a text box in the admin area where you can customize this. If the text box is empty, it should show the default message (Availability: In stock) but if the text box has a value it should show the text box value instead. This should also be for Configurable Products, but only the main Configurable Product will have the Availability message be able to be customized.