This reopens https://github.com/apache/incubator-spark/pull/640 against the new repo

Author: Sandy Ryza <sandy@cloudera.com>

Closes #30 from sryza/sandy-spark-1004 and squashes the following commits:

89889d4 [Sandy Ryza] Move unzipping py4j to the generate-resources phase so that it gets included in the jar the first time
5165a02 [Sandy Ryza] Fix docs
fd0df79 [Sandy Ryza] PySpark on YARN