Model_Soft and Model_Temporal purge cascade

[sagikazarmark]

The purge method in the mentioned Models do not have the option, to cascade delete the related models, but I think it should have.

I mention #297 here. Model_Soft doesn't have the option to permanently delete an entity, so it won't work without that.

So purging in a Temporal/Soft model means purging in related Temporal/Soft models, and normal delete in the rest.

[emlynwest]

Model_Soft already has this functionality including cascading, looking at the code. For temporal this becomes a lot harder as all temporal relations would need to know they are being purged and not just deleted.