* Authentication
* Integrate server-session package? https://github.com/yesodweb/serversession/blob/master/README.md
We don't really use sessions at the moment, but this will be useful once there's a UI.
Alternately, need to look into JWT (http://jwt.io/) to figure out whether this approach
is relevant for us.
* Payouts
* Previously, I had thought it would be easiest for payments to be made directly to
a per-aftok BTC address, and a subsequent transaction used to then distribute
that transaction to the participants. However, I now think it makes more sense to
present the payer with a transaction to complete and sign that sends funds directly
from their wallet to the participants, as a multiparty txn requiring signatures
of both the aftok server (which would sign in advance) and the payer. This avoids
the central server even momentarily having control of any funds.