A jar file containing Python sources in it could be used as a Python package, just like zip file.

spark-submit already put the jar file into PYTHONPATH, this patch also put it in the sys.path, then it could be used in Python worker.

Author: Davies Liu <davies@databricks.com>

Closes #4652 from davies/jar and squashes the following commits:

17d3f76 [Davies Liu] support .jar as python package