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)
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