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.

tests/app/dao/test_rates_dao.py

@@ -0,0 +1,18 @@
+from datetime import datetime
+
+from decimal import Decimal
+
+from app.dao.rates_dao import get_rate_for_type_and_date
+
+
+def test_get_rate_for_type_and_date(notify_db):
+    rate = get_rate_for_type_and_date('sms', datetime.utcnow())
+    assert rate.rate == Decimal("1.58")
+
+    rate = get_rate_for_type_and_date('sms', datetime(2016, 6, 1))
+    assert rate.rate == Decimal("1.65")
+
+
+def test_get_rate_for_type_and_date_early_date(notify_db):
+    rate = get_rate_for_type_and_date('sms', datetime(2014, 6, 1))
+    assert not rate