Add folders support to Jenkins Job Builder

To provide support for managing large numbers of jobs across multiple teams within a single instance, jenkins-job-builder needs to be able to support uploading jobs to designated folders. This should include the capability to set an initial base folder from the JJB command line, and for jobs to specify folders that are used relative to the base. JJB should address nested folders via <folder>/<folder>/<job> and should not need to be aware of how this needs to be translated into a specific url path to perform the necessary request.