Added a new rates table.

We only need one rate per channel. This reflects that. The provider_rates has been left for now, it is still not being used.
New dao has been added to select the right rate for the given notification_type and date of notificaiton.

app/dao/rates_dao.py

@@ -0,0 +1,11 @@
+from sqlalchemy import desc
+
+from app import db
+from app.models import Rates
+
+
+def get_rate_for_type_and_date(notification_type, date_sent):
+    return db.session.query(Rates).filter(Rates.notification_type == notification_type,
+                                          Rates.valid_from <= date_sent
+                                          ).order_by(Rates.valid_from.desc()
+                                                     ).limit(1).first()