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.
As it stands, this statement won't delete resources and resources_revisions on CKAN 2.3. You need to copy the relevant statements to the `ImportError` section, or better yet only wrap the ResourceGroup related ones on the try/catch