Add CustomerShopper.corepos_customer_id and refactor importers

removes phone/email support for now..also change how we handle
default/empty values for member start/end date..

rattail_corepos/importing/model.py

@@ -64,6 +64,15 @@ class CustomerImporter(importing.model.CustomerImporter):
     }
 
 
+class CustomerShopperImporter(importing.model.CustomerShopperImporter):
+
+    extensions = {
+        '_corepos': [
+            'corepos_customer_id',
+        ],
+    }
+
+
 class MemberImporter(importing.model.MemberImporter):
 
     extensions = {