Deprecate recurring functionality

There is too much variance between the recurring functionality provided by gateways for ActiveMerchant to do a good job of abstracting it. Thus this deprecates all such functionality with the intention of stripping it out completely at some future point.

Note that this does *not* refer to tokenization support, but rather the ability to set up and manage the ability some gateways support to automatically charge a card on a recurring basis.

@bslobodin @odorcicd @jduff do you concur?