ENH: Add zipline.utils.cache.

Implements a `CachedObject` utility class for wrapping cached results
with an expiration date.

tests/utils/test_cache.py

@@ -0,0 +1,21 @@
+from unittest import TestCase
+
+from pandas import Timestamp, Timedelta
+
+from zipline.utils.cache import CachedObject, Expired
+
+
+class CachedObjectTestCase(TestCase):
+
+    def test_cached_object(self):
+        expiry = Timestamp('2014')
+        before = expiry - Timedelta('1 minute')
+        after = expiry + Timedelta('1 minute')
+
+        obj = CachedObject(1, expiry)
+
+        self.assertEqual(obj.unwrap(before), 1)
+        self.assertEqual(obj.unwrap(expiry), 1)  # Unwrap on expiry is allowed.
+        with self.assertRaises(Expired) as e:
+            obj.unwrap(after)
+        self.assertEqual(e.exception.args, (expiry,))