Add create_exchange method

This PR adds a method for creating an exchange by PUTing the `/api/exchanges/:vhost/:name` endpoint per [this document](http://hg.rabbitmq.com/rabbitmq-management/raw-file/3646dee55e02/priv/www-api/help.html).

It assumes the properties given by the example JSON for the attributes.

I'm hoping I didn't just miss the method (almost added a `create_binding` until I realized it existed), but let me know if I did.