Standalone middleware

In Rails 4 Rails::Rack::Logger has a different argument set and expects the tags for tagged logging if you enable tagged logging. If you try to use tagged logging following this README it will fail with an error where it can't find the tags. You can see the 2 conflicting methods [here](https://github.com/rails/rails/blob/master/railties/lib/rails/rack/logger.rb#L21) and [here](https://github.com/rails/rails/blob/master/railties/lib/rails/rack/logger.rb#L57).

All the middleware needs to do is set the log level before the Rails::Rack::Logger middleware is ran, no need to inherit from Rails::Rack::Logger.

/cc @wfarr