Provide option to skip queue declaration step for read-only clients

[achilleasa]

This PR introduces a boolean declare flag to the Channel.queue method. The flag is true by default but applications with read-only access to the queue can set it to false to skip the queue declaration step (which requires write-access) and get back a Queue instance that they can consume off.

Fixes #58

[coveralls]

Coverage increased (+0.008%) to 93.115% when pulling 48ec082 on allow-clients-with-ro-access-to-skip-resource-declarations into cb18a05 on master.