- 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.

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.

- Rework/expand the nav bar with more of the docs site
- Removing parts of docs about EC2 and Mesos that differentiate between
  running 0.5 and before
    - Merged subheadings from running-on-amazon-ec2.html that are still relevant
      (i.e., "Using a newer version of Spark" and "Accessing Data in S3") into
      ec2-scripts.html and deleted running-on-amazon-ec2.html
- Added some TODO comments to a few docs
- Updated the blurb about AMP Camp
- Renamed programming-guide to spark-programming-guide
- Fixing typos/etc. in Standalone Spark doc