Litle account updater support

 - Adds account updater XML to the commerce_litle_test module.
 - Some other improvements to the commerce_litle_test module.
 - Adds a hook to alter the end date when creating a sustainer series.
 - Adds a setting to Litle configuration to extend new sustainers by one month.
 - Adds functionality to the Litle module to update a sustainer series when Litle sends account updater information.
 - Refactored the billing update code so that Account Updater can use it.

Record the token tags sent in the request so we can pass them to the account updater response if needed and provide a token response.