Support event collector

[rectalogic]

We would like to use the event collector so that on exception we can log more information on what occurred.

Is the event collector (PBase.events) expected to be a RefreshDB implementation?
https://github.com/OpenIDC/pyoidc/blob/master/src/oic/oauth2/base.py#L92

RefreshDB is deprecated https://github.com/OpenIDC/pyoidc/blob/master/src/oic/utils/sdb.py#L278

Also at least one place passes an additional ref kwarg which RefreshDB.store() does not support

if self.events is not None:
    self.events.store("HTTP response", r, ref=url)

https://github.com/OpenIDC/pyoidc/blob/master/src/oic/oauth2/base.py#L140

[tpazderka]

PBase.events is not related to RefreshDB.

The events are used in test tool and you can probably take inspiration here.