Don't log passwords

Fixes https://github.com/gocardless/hutch/issues/79
Add channel_prefetch to config
Add a test for the channel_prefetch
Merge pull request #81 from drewish/do-not-log-passwords

Don't log passwords
Merge pull request #80 from drewish/add-channel_prefetch-to-config

Add channel_prefetch to config
Make basic.qos default value the same as in the protocol

References #80.