Add support for submitting "varref" and "comments" fields to Realex Payments

The Realex Payments gateway is implemented in CRM/Core/Payment/Realex.php. However, the Realex spec for the authorisation requests also allows the optional addition of "varref" and "comments" fields in the request, which currently isn't implemented in CiviCRM.

We had a client request that these fields be implemented for reporting purposes.

The "varref" field is used to send variable identifiers and are used to generate reports. The attached patch sets this variable to be the contribution type. This allows the client to separate out donation transactions from membership ones, etc within Realex's report generator.

The "comments" field is just used to send additional information about the transaction to Realex. The attached patch sets this field to be the description and the contribution page id. This information is visible in Realex when the user clicks in to see the full detail of an individual transaction.