Use Bootstrap progress bars in web UI
Merge branch 'master' of https://github.com/mesos/spark into bootstrap-update

cially if it merges an updated upstream into a topic branch.
Added started tasks to progress bar
Merge branch 'master' of https://github.com/mesos/spark into bootstrap-update

Conflicts:
	core/src/main/scala/spark/ui/jobs/IndexPage.scala
Fixed continuity of executorToTasksActive, changed color of progress bars
Removed intermediate set for activeTasks, removed progress bar margin
Fixed style, lowered height of progress bars
Fixed wrap style
Merge pull request #744 from karenfeng/bootstrap-update

Use Bootstrap progress bars in web UI