hook to handle the access token response

[imhoffd]

I needed to be able to alter the response of the access token before it was sent. I think this is a sufficient patch.

[thomseddon]

This is turning out to be quite a common request, but I think i'd prefer a more general solution like #115

[imhoffd]

I actually don't need this feature anymore and looking back I would have done more to make it more general as you have outlined. Cheers!