have the logging middleware set the rack logger for each request env

This updates Deas' logging middleware to have it set the logger for
the rack env of each request.  This ensures that logger calls in
the template scope (which use Sinatra's `logger` helper) use the
configured logger (if any).

Previously, b/c we are setting Sinatra's `:logging` config to `false`,
Sinatra was setting `env['rack.logger'] to some rack null-logger.
Sinatra's `logger` template helper method just uses whatever is in
`env['rack.logger']` so logging in templates would always use this
null-logger.  This ensures logging in templates now always uses Deas'
configured logger.

@jcredding ready for review.