eWay Rapid: Tweak authorization support

We now pass a "Method" of "Authorise" for an authorization as per this
documentation:

http://api-portal.anypoint.mulesoft.com/eway/api/eway-rapid-31-api/docs/reference/preauth#Authorisation

A customer reported that an authorize was showing up as a purchase.
This is an attempt to address that.  The sandbox doesn't seem to respond
differently for this "Method" change but I'm hopeful that production
will see a difference.  Either way, this brings us more inline with the
current eWay Rapid docs.
Litle: Add support for dynamic descriptors

This allows customization of what appears on a customer's statement on a
per transaction basis.
Litle: Make descriptors more consistent

Dynamic descriptors are pretty much all over the map across gateways in
terms of attribute names and behaviors.  This makes the properties
consistent with at least one other gateway (Braintree Blue).

Perhaps somewhere down the road we'll start to see more commonalities
between gateways and find a better abstraction.
DIBS Payment Services

This commit is maintained separate until DIBS setup a sandbox.
Code by @maxwhite