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