macaroon: use slice not map in Verify

This makes it possible to have several possible
discharge macaroons with the same id.
macaroon: add TestVerify
Merge pull request #5 from rogpeppe/004-discharge-allow-many

macaroon: allow many discharge macaroons for a given id
update README
Merge pull request #6 from rogpeppe/005-update-README

update README
Merge branch 'master' of github.com:rogpeppe/macaroon into 006-remove-TODOs
update README.md