The new package contains a new interface (`Challenger`) that any
new challenger must implement.
Because aperture uses the new interface instead of using directly the
`LndChallenger` struct I added the `Stop()` method to the
`mint.Challenger`. Instead of also adding the `Start()` method the constructor
returns a Challenger already "started".
This package adheres to the agreed upon internal design document of the
macaroon portion of an LSAT. It is able to mint LSATs for a set of
services at any tier, each containing their desired set of constraints.
LSAT verification so far only ensures the that token was minted by us
and that the target service attempted to be accessed is authorized
according to the white-listed services contained in the token.
