Extensions api - handle renewal payments that fail

Attached are some basic api functions for handling extensions.

With a default install and this patch there is a hard coded extension of two reattempts on a failed renewal:
* The first failed payment extends for 3 days
* The second failed attempt extends for 5 days
* The third attempt will expire the recurring subscription.

What is still needed is a UI for allowing site admin's to change the default extension periods as well as the ability to override these for individual recurring fees. This will mean that you can define exactly what happens on any specific recurring fee failing and how many re-attempt should be made until giving up.

Attach is what I believe is a mostly complete attempt at a basic extension system.

The general idea of this feature is when a payment fails the recurring fee can be extended for some period (e.g. to allow person to update CC details or put funds on their card). Then uc_recurring will retry the renewal again after that period. The idea is you can configure what these intervals will be and how many times it should try until giving up and cancelling the subscription.

There are defaults extensions that will be used for all products, but the system has been built in a way that allows for extensions to be configured for each individual recurring fee as well. At the moment I haven't built the UI for being able to change these in each product but that can be done later.

There are also api function that other external gateways can used to tell uc_recurring that they are extending the subscription

function uc_recurring_extend_fee(&$fee, $extend_seconds);

Attached is an image of what the admin settings looks like to configure these extensions.

realized that extensions should not be applied to orders where the recurring fee is handled by its own gateway

Ok cleaned up a couple of small things and committed this features.