WorkUnit and RunTracker concepts for pants.

WorkUnit represents a unit of work, for the purpose of tracking and timing.

RunTracker keeps track of all the work in a single pants run.

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A periodic thread.

Runs the thread's action at a fixed time interval.

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A clarifying comment.
A Reporter class and a ConsoleReporter implementation.

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Merge branch 'master' into new_reporting

Conflicts:
	src/python/twitter/common/threading/periodic_thread.py
	src/python/twitter/pants/goal/run_tracker.py
WIP