402 Handshake

    GET/HEAD http://api.21.co/<endpoint>
        -> No body.

        <- 402 - Payment Required.
        Price: 50
        Bitcoin-Payment-Channel-Server: http://payment.21.co/

    GET/etc. http://api.21.co/<endpoint>
        Bitcoin-Payment-Channel-Token: d47e9ad295f790a3d8254092fa3a02d4478070ba25b2c6812736390e8e9d963f

        <- 200 - OK.
        <resource>
