afamqp: Make the username() and password() options mandatory

For the destination to work, we need both username() and password()
supplied, otherwise the underlying library will crash. If either of
those is unset, fail the driver initialization. This fixes #80.

Signed-off-by: Gergely Nagy <algernon@balabit.hu>
Merge pull request #107 from algernon/h/amqp/userpass-mandatory

afamqp: Make the username() and password() options mandatory