Don't log passwords

Fixes https://github.com/gocardless/hutch/issues/79

New output looks like:
```
2014-01-27T06:30:46Z 79291 INFO -- connecting to rabbitmq (amqp://guest@localhost:5672/)
2014-01-27T06:30:46Z 79291 INFO -- opening rabbitmq channel
2014-01-27T06:30:46Z 79291 INFO -- using topic exchange 'zoinks'
2014-01-27T06:30:46Z 79291 INFO -- connecting to rabbitmq HTTP API (http://guest@localhost:15672/)
```