Add ability to pass several signals to `receiver`

[dmedvinsky]

Pull Request: ​https://github.com/django/django/pull/136

This adds ability to use receiver in the following way:

@receiver([post_save, post_delete], sender=MyModel)
def signals_receiver(sender, **kwargs):
    ...

Quite often I want to connect both post_save and post_delete signals
for some particular sender.

[Florian Apolloner <florian@…>]

In [d4da08375b634544b95859d4d4667b8f05e3a29a]:

Fixed #18454 -- Added ability to pass a list of signals to receiver.

Added ability to use receiver decorator in the following way:

@receiver([post_save, post_delete], sender=MyModel)
def signals_receiver(sender, kwargs):

...