Use background threads for Rabbit I/O

We've run into a bug locally when trying to use this appender with an NServiceBus service, where because the RabbitMQ client uses foreground threads for I/O our service installer never finishes... Luckily, RabbitMQ client 3.4.0 has an option to use background threads instead (https://github.com/rabbitmq/rabbitmq-dotnet-client/pull/22), and this pull request enables that.

Sadly RabbitMQ.Client 3.4.0 isn't on NuGet.org yet (so this works for me locally with a non-public NuGet server), but should but should be very shortly... (https://groups.google.com/forum/#!topic/rabbitmq-users/AJl3VnIKjq4)