Organize configuration docs

This PR improves and organizes the config option page
and makes a few other changes to config docs. See a preview here:
http://people.apache.org/~pwendell/config-improvements/configuration.html

The biggest changes are:
1. The configs for the standalone master/workers were moved to the
standalone page and out of the general config doc.
2. SPARK_LOCAL_DIRS was missing from the standalone docs.
3. Expanded discussion of injecting configs with spark-submit, including an
example.
4. Config options were organized into the following categories:
- Runtime Environment
- Shuffle Behavior
- Spark UI
- Compression and Serialization
- Execution Behavior
- Networking
- Scheduling
- Security
- Spark Streaming

Author: Patrick Wendell <pwendell@gmail.com>

Closes #880 from pwendell/config-cleanup and squashes the following commits:

93f56c3 [Patrick Wendell] Feedback from Matei
6f66efc [Patrick Wendell] More feedback
16ae776 [Patrick Wendell] Adding back header section
d9c264f [Patrick Wendell] Small fix
e0c1728 [Patrick Wendell] Response to Matei's review
27d57db [Patrick Wendell] Reverting changes to index.html (covered in #896)
e230ef9 [Patrick Wendell] Merge remote-tracking branch 'apache/master' into config-cleanup
a374369 [Patrick Wendell] Line wrapping fixes
fdff7fc [Patrick Wendell] Merge remote-tracking branch 'apache/master' into config-cleanup
3289ea4 [Patrick Wendell] Pulling in changes from #856
106ee31 [Patrick Wendell] Small link fix
f7e79bc [Patrick Wendell] Re-organizing config options.
54b184d [Patrick Wendell] Adding standalone configs to the standalone page
592e94a [Patrick Wendell] Stash
29b5446 [Patrick Wendell] Better discussion of spark-submit in configuration docs
2d719ef [Patrick Wendell] Small fix
4af9e07 [Patrick Wendell] Adding SPARK_LOCAL_DIRS docs
204b248 [Patrick Wendell] Small fixes
(cherry picked from commit 7801d44fd3bcf4d82e6db12574cc42bef15bf0e1)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
Spark 1916

The changes could be ported back to 0.9 as well.
Changing in.read to in.readFully to read the whole input stream rather than the first 1020 bytes.
This should ok considering that Flume caps the body size to 32K by default.

Author: David Lemieux <david.lemieux@radialpoint.com>

Closes #865 from lemieud/SPARK-1916 and squashes the following commits:

a265673 [David Lemieux] Updated SparkFlumeEvent to read the whole stream rather than the first X bytes.
[SPARK-1712]: TaskDescription instance is too big causes Spark to hang

Author: witgo <witgo@qq.com>

Closes #694 from witgo/SPARK-1712_new and squashes the following commits:

0f52483 [witgo] review commit
83ce29b [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
52e6752 [witgo] reset test SparkContext
63636b6 [witgo] review commit
44a59ee [witgo] review commit
3b6d48c [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
926bd6a [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
9a5cfad [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
03cc562 [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
b0930b0 [witgo] review commit
b1174bd [witgo] merge master
f76679b [witgo] merge master
689495d [witgo] fix scala style bug
1d35c3c [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
062c182 [witgo] fix small bug for code style
0a428cf [witgo] add unit tests
158b2dc [witgo] review commit
4afe71d [witgo] review commit
9e4ffa7 [witgo] review commit
1d35c7d [witgo] fix hang
7965580 [witgo] fix Statement order
0e29eac [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
3ea1ca1 [witgo] remove duplicate serialize
743a7ad [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
86e2048 [witgo] Merge branch 'master' of https://github.com/apache/spark into SPARK-1712_new
2a89adc [witgo] SPARK-1712: TaskDescription instance is too big causes Spark to hang

(cherry picked from commit 4dbb27b0cf4eb67c92aad2c1158616312f5a54e6)
Signed-off-by: Matei Zaharia <matei@databricks.com>
Added doctest and method description in context.py

Added doctest for method textFile and description for methods _initialize_context and _ensure_initialized in context.py

Author: Jyotiska NK <jyotiska123@gmail.com>

Closes #187 from jyotiska/pyspark_context and squashes the following commits:

356f945 [Jyotiska NK] Added doctest for textFile method in context.py
5b23686 [Jyotiska NK] Updated context.py with method descriptions

(cherry picked from commit 9cff1dd25abc5e848720d853172ed42e35376fd0)
Signed-off-by: Matei Zaharia <matei@databricks.com>