Proposal for incoming notifications

This is a proposal to handle incoming notifications from payment gateways in a generic way (where the payment gateway notifies the merchant's backend server directly). Some gateways are currently using completePurchase() for this purpose but it's actually not its original intent, judging from official examples and documentation.

I have the PHP interface code for this ready as well, in https://github.com/sergej-koscejev/omnipay-common, but it's rather trivial so I'm only submitting the changes to the documentation at this time.

What do you think?