logic.action.update.harvest_source_clear supports retiring of ResourceGroup

Makes ckanext-harvest compatible with both CKAN 2.2 and 2.3. Toggles offending SQL statements referring to `resource_group` depending on existence of `ckan.model.ResourceGroup`. Fixes #114 Also: PEP8.
of course - I'll add delete statements for resource and resource_revision to the v2.3 "except" block.