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
Style cleanup based on Matei feedback
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.
Merge branch 'master' of https://github.com/mesos/spark into master-csd