ENH: Add expiring cache.

Add a cache interface which supports expirable entries with a changeable
backend for the cache into which they are entered.

The default cache is a `dict` but could swapped for
`cachetools.LRUCache` or any other cache which supports `__get__`,
`__set__`, and `__del__`.

So that consumers can change the use of `CachedObjects` stored in a
cache from:

```
self._cache = {}

...

try:
    obj = self._cache[key]
    try:
        return obj.unwrap(dt)
    except Expired:
        pass
except KeyError:
    pass

...

self._cache[key] = CachedObject(value, new_expiration)
```

to:

```
self._cache = ExpiringCache(LRUCache(maxsize=6))

...

try:
    return self._cache.get(key, dt)
except KeyError:
    # Get fresh value
    ...

    self._cache.set(key, value, new_expiration)
```

tests/utils/test_cache.py

@@ -2,7 +2,7 @@ from unittest import TestCase
 
 from pandas import Timestamp, Timedelta
 
-from zipline.utils.cache import CachedObject, Expired
+from zipline.utils.cache import CachedObject, Expired, ExpiringCache
 
 
 class CachedObjectTestCase(TestCase):
@@ -19,3 +19,42 @@ class CachedObjectTestCase(TestCase):
         with self.assertRaises(Expired) as e:
             obj.unwrap(after)
         self.assertEqual(e.exception.args, (expiry,))
+
+
+class ExpiringCacheTestCase(TestCase):
+
+    def test_expiring_cache(self):
+        expiry_1 = Timestamp('2014')
+        before_1 = expiry_1 - Timedelta('1 minute')
+        after_1 = expiry_1 + Timedelta('1 minute')
+
+        expiry_2 = Timestamp('2015')
+        after_2 = expiry_1 + Timedelta('1 minute')
+
+        expiry_3 = Timestamp('2016')
+
+        cache = ExpiringCache()
+
+        cache.set('foo', 1, expiry_1)
+        cache.set('bar', 2, expiry_2)
+
+        self.assertEqual(cache.get('foo', before_1), 1)
+        # Unwrap on expiry is allowed.
+        self.assertEqual(cache.get('foo', expiry_1), 1)
+
+        with self.assertRaises(KeyError) as e:
+            self.assertEqual(cache.get('foo', after_1))
+        self.assertEqual(e.exception.args, ('foo',))
+
+        # Should raise same KeyError after deletion.
+        with self.assertRaises(KeyError) as e:
+            self.assertEqual(cache.get('foo', before_1))
+        self.assertEqual(e.exception.args, ('foo',))
+
+        # Second value should still exist.
+        self.assertEqual(cache.get('bar', after_2), 2)
+
+        # Should raise similar KeyError on non-existent key.
+        with self.assertRaises(KeyError) as e:
+            self.assertEqual(cache.get('baz', expiry_3))
+        self.assertEqual(e.exception.args, ('baz',))