Insert initial letter rates.

Create letter rates dao.
Query to fetch letter rates.

tests/app/dao/test_letter_rate_dao.py

@@ -0,0 +1,41 @@
+from datetime import datetime
+
+from decimal import Decimal
+
+from app.dao.letter_rate_dao import dao_create_letter_rate, get_letter_rates_for_daterange
+from app.models import LetterRate
+
+
+def test_dao_create_letter_rate(notify_db_session):
+    letter_rate = LetterRate(start_date=datetime(2017, 12, 1),
+                             rate=0.33,
+                             crown=True,
+                             sheet_count=1,
+                             post_class='second')
+
+    dao_create_letter_rate(letter_rate)
+
+    rates = LetterRate.query.all()
+    assert len(rates) == 1
+
+
+def test_get_letter_rates_for_daterange(notify_db_session):
+    letter_rate = LetterRate(start_date=datetime(2017, 12, 1),
+                             rate=0.33,
+                             crown=True,
+                             sheet_count=1,
+                             post_class='second')
+
+    dao_create_letter_rate(letter_rate)
+    letter_rate = LetterRate(start_date=datetime(2016, 12, 1),
+                             end_date=datetime(2017, 12, 1),
+                             rate=0.30,
+                             crown=True,
+                             sheet_count=1,
+                             post_class='second')
+
+    dao_create_letter_rate(letter_rate)
+
+    results = get_letter_rates_for_daterange(date=datetime(2017, 12, 2), crown=True, sheet_count=1, post_class='second')
+    assert len(results) == 1
+    assert results[0].rate == Decimal('0.33')