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
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.
More minor UI changes including code review feedback.
Merge pull request #833 from rxin/ui

Various UI improvements.
Rename `memoryBytesToString` and `memoryMegabytesToString`

These are used all over the place now and they are not specific to memory at all.

memoryBytesToString --> bytesToString
memoryMegabytesToString --> megabytesToString
Merge pull request #829 from JoshRosen/pyspark-unit-tests-python-2.6

Fix PySpark unit tests on Python 2.6
Merge pull request #836 from pwendell/rename

Rename `memoryBytesToString` and `memoryMegabytesToString`
Implementing SPARK-878 for PySpark: adding zip and egg files to context and passing it down to workers which add these to their sys.path