Add an event that tells how long run() took.

Quintessential example:

@luigi.Task.event_handler(luigi.Event.PROCESSING_TIME)
def be_mindful_of_time(task, x):
	my.metrics.send_value(type(task).__name__, x)
Make the test pass under py2.6 version of unittest.
Merge pull request #402 from upworthy/trigger-processing-time-event

Add an event that tells how long run() took.