This patch changes "yarn-standalone" to "yarn-cluster" (but still supports the former).  It also cleans up the Running on YARN docs and adds a section on how to view logs.

Author: Sandy Ryza <sandy@cloudera.com>

Closes #95 from sryza/sandy-spark-1197 and squashes the following commits:

563ef3a [Sandy Ryza] Review feedback
6ad06d4 [Sandy Ryza] Change yarn-standalone to yarn-cluster and fix up running on YARN docs