fix tails instructions

Apparently APT has a bug with the slash notation for installing packages from
specific suites.
Merge pull request #121 from leif/master

fix tails instructions
add external command hook POND_HOOK_RECEIVE

Perhaps other command hooks should be added. This feels out of place in
network.go.
show empty (ack) messages in outbox

so they can be deleted when DELIVERY_SIGNATURE_INVALID happens