tag first release candidate of log4net 1.2.13

git-svn-id: https://svn.apache.org/repos/asf/logging/log4net/tags/1.2.13RC1@1539449 13f79535-47bb-0310-9956-ffa450edef68
merge version number and release note changes

git-svn-id: https://svn.apache.org/repos/asf/logging/log4net/branches/log4net-1.2.x@1539921 13f79535-47bb-0310-9956-ffa450edef68