SPARK-1474: Spark on yarn assembly doesn't include AmIpFilter

We use org.apache.hadoop.yarn.server.webproxy.amfilter.AmIpFilter in spark on yarn but are not included it in the assembly jar.

I tested this on yarn cluster by removing the yarn jars from the classpath and spark runs fine now.

Author: Thomas Graves <tgraves@apache.org>

Closes #406 from tgravescs/SPARK-1474 and squashes the following commits:

1548bf9 [Thomas Graves] SPARK-1474: Spark on yarn assembly doesn't include org.apache.hadoop.yarn.server.webproxy.amfilter.AmIpFilter
Fix two download suggestions in the docs:

1) On the quick start page provide a direct link to the downloads (suggested by @pbailis).
2) On the index page, don't suggest users always have to build Spark, since many won't.

Author: Patrick Wendell <pwendell@gmail.com>

Closes #662 from pwendell/quick-start and squashes the following commits:

0622f27 [Patrick Wendell] Fix two download suggestions in the docs: