Add missing `pass` keyword

I have no idea why this compiled before.
Update documentation for Task.complete()

The old documentation was lying I believe, there is no "False only first
time" behavior.  I even wrote this small sample program.

    import luigi.interface
    import luigi.task

    luigi.interface.setup_interface_logging()

    class MyTask(luigi.task.Task):
        def run(self):
            print 'hi'

    my_task = MyTask()

    print my_task.complete()
    my_task.run()
    print my_task.complete()

Which gave:

    $ ipython arash_test.py
    /home/arash/spotify/repos/luigis/github_luigi/luigi/task.py:471: UserWarning: Task MyTask() without outputs has no custom complete() method
      warnings.warn("Task %r without outputs has no custom complete() method" % self)
    False
    hi
    False

Also removed the 2 year old TODO. It's hard to change semantics this far
up in the project I think.
Simplify Task.complete()

Just using standard functional tools. Note that it's necessary to use
itertools to not change the semantics, in case one of the exists()
methods have side effects.
Merge pull request #526 from Tarrasch/simplify_task_complete

Simplify task complete