Better name for publish method

[Lukasz Balcerzak]

Currently we are using publish name for storing events as it is used directly from eventsourcing library. This might be misleading however - it's actually storing an event. We should call it simply store_event instead.

Not sure if we want to keep old method. djangoevents is not mature yet so we can change API without deprecating it first. OTOH, we might want to inform all users by issuing deprecationwarning (https://docs.python.org/3/library/warnings.html#default-warning-filters).