Update to 1.1.1

* Updated GoCardless API to v0.4.2
* Added config option "PreAuth Only". This option is mutually exclusive
with "one off only", and will result in a pre-authorization agreement
beign created for all payments, including one off payments. Leave
unchecked to retain current operation where a pre-auth is only created
for
* Multiple changes to the _link and _capture functions to avoid
erroneously sending "Credit Card Payment Failed" email alerts when a
payment is still pending
* Added WHMCS Client Custom Field "GoCardless DD Auth ID" to support
client-wide direct debit authorisation.  Previously this was set at a
per service level. If a "Subscription ID" is set for a product/service,
this will override the client-wide authorisation ID.