Publisher confirms high level API

[botanicus]

Recently RabbitMQ introduced publisher confirms, which is basically a faster alternative to transactions.

Using publisher confirms requires a lot of client code though, we should come up with something more high level, so it'd be easier for people to use.

Irrelevant note: it's bug 24229 on RabbitMQ bugzilla (not publicly visible).

I've opened the same issue for the AMQP gem.