Gracefully close worker on term.

This change (attempts to) add a signal handler to the worker allowing it
to gracefully shutdown on a TERM signal. This means that it will finish
the tasks that it has accepted but not request any more work.
Make S3Target respect format
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.
Fix race condition in SnakebiteHdfsClient.mkdir()

Rewritten in EAFP style:
https://docs.python.org/2/glossary.html#term-eafp
Security fix
Merge pull request #404 from Tarrasch/fix-snakebite-mkdir-race

Fix race condition in SnakebiteHdfsClient.mkdir()
Allows specification of priority for hadoop jobs
Uses mkdir function in HdfsAtomicWritePipe init
Fixes a bug in graph creation for unknown dependencies
Merge pull request #402 from upworthy/trigger-processing-time-event

Add an event that tells how long run() took.
Bumped version to 1.0.17 and uploaded to PyPI
Fix scheduling tasks with many deps/params.
Merge pull request #409 from campanja/fix-large-rpc-requests

Fix scheduling tasks with many deps/params.
Fallback since older versions of Tornado can't handle max_hader_size
Merge pull request #399 from danielhfrank/graceful-shutdown

Gracefully close worker on term.
Merge pull request #401 from tommyengstrom/S3Target-format-fix

Make S3Target respect format
Merge pull request #406 from Houzz/mr_priority

Allows specification of priority for hadoop jobs
Merge pull request #408 from Houzz/fix_unknown_task_bug

Fixes a bug in graph creation for unknown dependencies
Revert "Gracefully close worker on term."