Fixes the class-level Exchange default initialization

I was trying to use the class-level `Exchange` default method but it was raising out because that compatibility module was included (not extended).  Rather than also include the module, I just pass the channel_or_connection down to the instance without coercion.

```ruby
> Bunny::Exchange.default(channel).publish("make clean", routing_key => "tasks")
NoMethodError: undefined method `channel_from' for Bunny::Exchange:Class
	from /Users/litch/.rvm/gems/ruby-2.1.1/gems/bunny-1.2.1/lib/bunny/exchange.rb:58:in `default'
	from (irb):9
	from /Users/litch/.rvm/rubies/ruby-2.1.1/bin/irb:11:in `<main>'
```