Bug fix for display of shuffle read/write metrics.

This fixes an error where empty cells are missing if a given task
has no shuffle read/write.
Capturing GC detials in TaskMetrics
Correct sorting order for stages
Correcting terminology in RDD page
Small style clean-up
Update to Mesos 0.12.1
Merge pull request #824 from mateiz/mesos-0.12.1

Update to Mesos 0.12.1
Set SPARK_CLASSPATH for maven repl tests
Merge pull request #825 from shivaram/maven-repl-fix

Set SPARK_CLASSPATH for maven repl tests
Fixed 2 bugs in executor UI.

1) UI crashed if the executor UI was loaded before any tasks started.
2) The total tasks was incorrectly reported due to using string (rather
than int) arithmetic.
Merge pull request #826 from kayousterhout/ui_fix

Fixed 2 bugs in executor UI (incl. SPARK-877)
Style cleanup based on Matei feedback
Fix PySpark unit tests on Python 2.6.
Merge pull request #822 from pwendell/ui-features

Adding GC Stats to TaskMetrics (and three small fixes)
A few small scheduler / job description changes.

1. Renamed SparkContext.addLocalProperty to setLocalProperty. And allow this function to unset a property.

2. Renamed SparkContext.setDescription to setCurrentJobDescription.

3. Throw an exception if the fair scheduler allocation file is invalid.
Renamed setCurrentJobDescription to setJobDescription.
Merge pull request #831 from rxin/scheduler

A few small scheduler / job description changes.
Various UI improvements.
Updated json output to allow for display of worker state

Ops teams need to ensure that the cluster is functional and performant.  Having to scrape the html source for worker state won't work reliably, and will be slow.  By exposing the state in the json output, ops teams are able to ensure a fully functional environment by querying for the json output and parsing for dead nodes.
More minor UI changes including code review feedback.