Support offsite gateways

[xdmx]

The current gem doesn't support the integration of offsite gateways (paypal / gocardless) and their required fields (redirect_url, callback_url)

[eric-smartlove]

As 'spreedly-core-ruby' gem is deprecated (his issues section has even just been removed), supporting offsite gateways (or documenting this support if it is already available) seems more and more important.

Developpers using offsite gateways are stuck with 'spreedly-core-ruby' and, incidentally, with Rails 3.x.

[joeybeninghove]

It looks like this might not get added considering the age of the issue, but it's a must have for us, so I'm going to be forking this and adding offsite support to it, just an FYI.

[duff]

Sounds good @joeybeninghove. We'd love to see how things go and we'd certainly consider merging in a pull request.

[joeybeninghove]

Here she is: #42