Allow logging to be configured to allow for async appenders in drop wizard

Hi there

Sometimes (not always but sometimes), we write services that take a lot of requests, it is a business requirement that they don't take very long.

We have been looking and playing with dropwizard as a nice platform for cranking out services, but we have had situations where logging can stall requests.

This pull makes it possible to flip the dropwizard appenders over to async mode