SPARK-1911: Emphasize that Spark jars should be built with Java 6.

This commit requires the user to manually say "yes" when buiding Spark
without Java 6. The prompt can be bypassed with a flag (e.g. if the user
is scripting around make-distribution).

Author: Patrick Wendell <pwendell@gmail.com>

Closes #859 from pwendell/java6 and squashes the following commits:

4921133 [Patrick Wendell] Adding Pyspark Notice
fee8c9e [Patrick Wendell] SPARK-1911: Emphasize that Spark jars should be built with Java 6.