[SPARK-5811] Added documentation for maven coordinates and added Spark Packages support

Documentation for maven coordinates + Spark Package support. Added pyspark tests for `--packages`

Author: Burak Yavuz <brkyvz@gmail.com>
Author: Davies Liu <davies@databricks.com>

Closes #4662 from brkyvz/SPARK-5811 and squashes the following commits:

56ccccd [Burak Yavuz] fixed broken test
64cb8ee [Burak Yavuz] passed pep8 on local
c07b81e [Burak Yavuz] fixed pep8
a8bd6b7 [Burak Yavuz] submit PR
4ef4046 [Burak Yavuz] ready for PR
8fb02e5 [Burak Yavuz] merged master
25c9b9f [Burak Yavuz] Merge branch 'master' of github.com:apache/spark into python-jar
560d13b [Burak Yavuz] before PR
17d3f76 [Davies Liu] support .jar as python package
a3eb717 [Burak Yavuz] Merge branch 'master' of github.com:apache/spark into SPARK-5811
c60156d [Burak Yavuz] [SPARK-5811] Added documentation for maven coordinates