Helpers for working on a bundle of macaroons

[btoews]

These lines are repeated a lot

macaroon.Parse(...)
...

macaroon.FindPermissionAndDischargeTokens(....)
...

dischargeByTicket := ...

We should do a quick survey of the boilerplate steps we're repeating when starting with an authorization header and wanting to process the macaroons within. We should come up with an API for simplifying these.