Common logging configuration across Rake/Rack.

We can enable verbose logging using `rake -v` or `rackup -d -p <port>`.
Change the default logging level to `warn`.

Info level is too verbose to be a default: for example, it records every 50
documents repopulated during an index migration.
Merge pull request #121 from alphagov/logging_config

Common logging configuration across Rake/Rack.