Expose 'note' field in PaypalExpressResponse

Extract  the `note_text` field from the Paypal response and expose it as `note` in the ruby `PaypalExpressResponse` object.

I had to update the `successful_details_response` fixture a little bit to match what I was actually receiving from Paypal. I am uncertain if we should at some point start having a remote integration test that validates what we are actually receiving.

@girasquid @karlhungus