Add support for HDFS artifacts, task namespacing and cross-config file dependencies

Hi Tim

here are my changes:
 * HDFS artifacts can be checked for using HdfsArtifact (webhdfs: URLs)  -- requires your cluster to have WebHDFS enabled
 * multiple config files can now be specified on the command line
 * when multiple config files are specified the task to be run must be prefixed with a namespace, eg. 'testns.task1'
 * dependencies can be made from tasks in one config to another -- requires namespacing

cheers
Paul