Add OrderHistory class

- the main purpose of this class is to manage querying the order totals
  from the database and slice various kinds of time series out of the
  data
- the class holds the former `aggregate_orders()` function as a method
- modularize the corresponding tests
- add `tests.config` with globals used when testing to provide a
  single source of truth for various settings

tests/config.py

@@ -0,0 +1,10 @@
+"""Globals used when testing."""
+
+
+# The day on which most test cases take place.
+YEAR, MONTH, DAY = 2016, 7, 1
+
+# Default time steps, for example, for `OrderHistory` objects.
+LONG_TIME_STEP = 60
+SHORT_TIME_STEP = 30
+TIME_STEPS = (SHORT_TIME_STEP, LONG_TIME_STEP)