worker: Fix the logic of saving the output of stages.

Do not save the output of the job in the python library.  If the save_results
is set, the outputs of the job are already saved.
worker: Set the save_info in the parent Worker class.

This lets both the pipeline and the classic worker use it.
hdfs: Use a unique name for different parts of the output of a job.
worker: Add a missing import to task_io.
jobpack: Add the save_results to the jobpack in the Parent worker.
Examples: Add a version of wiki reader that saves the output.
Merge pull request #483 from pooya/save_results

Save results