Allow to configure event store table name

[lukaszb]

Currently we have hardcoded event_journal name for the table (https://github.com/ApplauseOSS/djangoevents/blob/devel/djangoevents/models.py#L19). As not default value might be surprising for library users. Let's use default name (djangoevents_event) but allow to configure this.

Tasks

• Create djangoevents.config module which should read DJANGOEVENTS_CONFIG variable from settings.py
• Have a default djangoevents_event for table name
• Use table name from configuration at the model's Meta
• Use table name from configuration at the migration
• Document this at the readme
• Updated changelog and let users know of potentially breaking change (they would need to manually rename table or set table name to event_journal