Demote URL-specific messages to debug level

Demote log messages that are URL-specific to the `DEBUG` log level.

This enables us to output more useful `INFO` messages without polluting
the logs with unnecessary noise.
Make INFO the default log level

Make `INFO` the default log level and remove the `-debug` flag.

Previously, the `INFO` level was very noisy and so it was undesirable to
use this as the default. In the previous commit, many noisy log messages
were demoted to `DEBUG` level, which makes `INFO` much more usable.

As a consequence, the `-debug` flag is no longer necessary. `-verbose`
will now show `DEBUG` messages and `INFO` log messages will be shown by
default.

Notable examples of `INFO` messages include the AMQP and Redis
connection messages shown when the application initialises.
Merge pull request #77 from alphagov/demote-log-messages

Demote log messages and make `INFO` the default log level