YARN
- SparkPi was updated to not take in master as an argument; we should update the docs to reflect that.
- The default YARN build guide should be in maven, not sbt.
- This PR also adds a paragraph on steps to debug a YARN application.

Standalone
- Emphasize spark-submit more. Right now it's one small paragraph preceding the legacy way of launching through `org.apache.spark.deploy.Client`.
- The way we set configurations / environment variables according to the old docs is outdated. This needs to reflect changes introduced by the Spark configuration changes we made.

In general, this PR also adds a little more documentation on the new spark-shell, spark-submit, spark-defaults.conf etc here and there.

Author: Andrew Or <andrewor14@gmail.com>

Closes #701 from andrewor14/yarn-docs and squashes the following commits:

e2c2312 [Andrew Or] Merge in changes in #752 (SPARK-1814)
25cfe7b [Andrew Or] Merge in the warning from SPARK-1753
a8c39c5 [Andrew Or] Minor changes
336bbd9 [Andrew Or] Tabs -> spaces
4d9d8f7 [Andrew Or] Merge branch 'master' of github.com:apache/spark into yarn-docs
041017a [Andrew Or] Abstract Spark submit documentation to cluster-overview.html
3cc0649 [Andrew Or] Detail how to set configurations + remove legacy instructions
5b7140a [Andrew Or] Merge branch 'master' of github.com:apache/spark into yarn-docs
85a51fc [Andrew Or] Update run-example, spark-shell, configuration etc.
c10e8c7 [Andrew Or] Merge branch 'master' of github.com:apache/spark into yarn-docs
381fe32 [Andrew Or] Update docs for standalone mode
757c184 [Andrew Or] Add a note about the requirements for the debugging trick
f8ca990 [Andrew Or] Merge branch 'master' of github.com:apache/spark into yarn-docs
924f04c [Andrew Or] Revert addition of --deploy-mode
d5fe17b [Andrew Or] Update the YARN docs