Moving gwt logging over from the IO branch. Changed the gwt.logging
property to gwt.logging.enabled so that it doesn't conflict for projects
using the incubator which already defines gwt.logging

Review by: spoon@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@8260 8db76d5a-ed1c-0410-87a9-c151d255dfc7