Log4net 1.2.10 support

Because of the well-known horrible break in log4net between 1.2.10 and 1.2.11+, projects that rely on 1.2.10 can't use log4net.RabbitMQ because it uses later versions. This pull request makes a new package log4net.1.2.10.RabbitMQAppender that makes a 1.2.10-compatible version of log4net.RabbitMQAppender.