- Added a StorageLevels class for easy access to StorageLevel constants
  in Java
- Added doc comments on Function classes in Java
- Updated Accumulator and HadoopWriter docs slightly

- Edited quick start and tuning guide to simplify them a little
- Simplified top menu bar
- Made private a SparkContext constructor parameter that was left as
  public
- Various small fixes

also adding the version to the navbar so it is easy to tell which
version of Spark these docs were compiled for.

scaladoc (i.e. run `SKIP_SCALADOC=1 jekyll`).

throughout the docs: SPARK_VERSION, SCALA_VERSION, and MESOS_VERSION.

To use them, e.g. use {{site.SPARK_VERSION}}.

Also removes uses of {{HOME_PATH}} which were being resolved to ""
by the templating system anyway.

menu items that was causing the dropdowns to go away when the user
moved their mouse down towards them.

instead of the maximum number of outstanding fetches. This should make
it faster when there are many small map output files, as well as more
robust to overallocating memory on large map outputs.

1. Slight change in organization
2. Added pre-requisites
3. Made a new section about determining memory footprint
   of an RDD
4. Other small changes

This commit includes a quick start example that covers:
1) Basic usage of the Spark shell
2) A simple Spark job in Scala
3) A simple Spark job in Java