HTTP::Server: change default logging to INFO.

No need to clutter STDOUT by default; make `h.cr` accept `DEBUG`
environment variable instead to enable verbose output.

src/server.cr

@@ -24,7 +24,7 @@ module HTTP
 
     def logger
       @logger ||= Logger.new(STDOUT).tap do |logger|
-        logger.level = Logger::Severity::DEBUG
+        logger.level = Logger::Severity::INFO
         logger.formatter = Logger::Formatter.new do |s, d, p, message, io|
           io << message
         end