[SPARK-2860][SQL] Fix coercion of CASE WHEN.

Author: Michael Armbrust <michael@databricks.com>

Closes #1785 from marmbrus/caseNull and squashes the following commits:

126006d [Michael Armbrust] better error message
2fe357f [Michael Armbrust] Fix coercion of CASE WHEN.
[SPARK-2859] Update url of Kryo project in related docs

JIRA Issue: https://issues.apache.org/jira/browse/SPARK-2859

Kryo project has been migrated from googlecode to github, hence we need to update its URL in related docs such as tuning.md.

Author: Guancheng (G.C.) Chen <chenguancheng@gmail.com>

Closes #1782 from gchen/kryo-docs and squashes the following commits:

b62543c [Guancheng (G.C.) Chen] update url of Kryo project
SPARK-2380: Support displaying accumulator values in the web UI

This patch adds support for giving accumulators user-visible names and displaying accumulator values in the web UI. This allows users to create custom counters that can display in the UI. The current approach displays both the accumulator deltas caused by each task and a "current" value of the accumulator totals for each stage, which gets update as tasks finish.

Currently in Spark developers have been extending the `TaskMetrics` functionality to provide custom instrumentation for RDD's. This provides a potentially nicer alternative of going through the existing accumulator framework (actually `TaskMetrics` and accumulators are on an awkward collision course as we add more features to the former). The current patch demo's how we can use the feature to provide instrumentation for RDD input sizes. The nice thing about going through accumulators is that users can read the current value of the data being tracked in their programs. This could be useful to e.g. decide to short-circuit a Spark stage depending on how things are going.

![counters](https://cloud.githubusercontent.com/assets/320616/3488815/6ee7bc34-0505-11e4-84ce-e36d9886e2cf.png)

Author: Patrick Wendell <pwendell@gmail.com>

Closes #1309 from pwendell/metrics and squashes the following commits:

8815308 [Patrick Wendell] Merge remote-tracking branch 'apache/master' into HEAD
93fbe0f [Patrick Wendell] Other minor fixes
cc43f68 [Patrick Wendell] Updating unit tests
c991b1b [Patrick Wendell] Moving some code into the Accumulators class
9a9ba3c [Patrick Wendell] More merge fixes
c5ace9e [Patrick Wendell] More merge conflicts
1da15e3 [Patrick Wendell] Merge remote-tracking branch 'apache/master' into metrics
9860c55 [Patrick Wendell] Potential solution to posting listener events
0bb0e33 [Patrick Wendell] Remove "display" variable and assume display = name.isDefined
0ec4ac7 [Patrick Wendell] Java API's
e95bf69 [Patrick Wendell] Stash
be97261 [Patrick Wendell] Style fix
8407308 [Patrick Wendell] Removing examples in Hadoop and RDD class
64d405f [Patrick Wendell] Adding missing file
5d8b156 [Patrick Wendell] Changes based on Kay's review.
9f18bad [Patrick Wendell] Minor style changes and tests
7a63abc [Patrick Wendell] Adding Json serialization and responding to Reynold's feedback
ad85076 [Patrick Wendell] Example of using named accumulators for custom RDD metrics.
0b72660 [Patrick Wendell] Initial WIP example of supporing globally named accumulators.
SPARK-1680: use configs for specifying environment variables on YARN

Note that this also documents spark.executorEnv.*  which to me means its public.  If we don't want that please speak up.

Author: Thomas Graves <tgraves@apache.org>

Closes #1512 from tgravescs/SPARK-1680 and squashes the following commits:

11525df [Thomas Graves] more doc changes
553bad0 [Thomas Graves] fix documentation
152bf7c [Thomas Graves] fix docs
5382326 [Thomas Graves] try fix docs
32f86a4 [Thomas Graves] use configs for specifying environment variables on YARN