Implement store for Flo2Cash

[bruno]

Implements tokenisation for Flo2Cash. Adds support for store, unstore and tests for purchase with tokenised card.

Flo2Cash has two methods to tokenise, AddCard and AddCardWithUniqueReference, we implement one or the other based on the presence of options[:order_id]. The main difference between both is simply that one allows the unique reference to be associated to the token, which can then be used by the merchant to search for the token using that reference, the other just creates a simple token.

Refactor the purchase method to use the ProcessPurchase at the suggestion from support at Flo2Cash since most of their merchants are not using ProcessAuthorise (that's what I have been told).

Note: I am not entirely sure why the purchase was original set up as auth + capture so please feel free to comment on that.