[SPARK-11978][ML] Move dataset_example.py to examples/ml and rename to dataframe_example.py

Since ```Dataset``` has a new meaning in Spark 1.6, we should rename it to avoid confusion.
#9873 finished the work of Scala example, here we focus on the Python one.
Move dataset_example.py to ```examples/ml``` and rename to ```dataframe_example.py```.
BTW, fix minor missing issues of #9873.
cc mengxr

Author: Yanbo Liang <ybliang8@gmail.com>

Closes #9957 from yanboliang/SPARK-11978.