Added HStoreField.

Thanks to `django-hstore` for inspiration in some areas, and many people
for reviews.

docs/ref/contrib/postgres/validators.txt

@@ -0,0 +1,20 @@
+==========
+Validators
+==========
+
+.. module:: django.contrib.postgres.validators
+
+``KeysValidator``
+-----------------
+
+.. class:: KeysValidator(keys, strict=False, messages=None)
+
+    Validates that the given keys are contained in the value. If ``strict`` is
+    ``True``, then it also checks that there are no other keys present.
+
+    The ``messages`` passed should be a dict containing the keys
+    ``missing_keys`` and/or ``extra_keys``.
+
+    .. note::
+        Note that this checks only for the existence of a given key, not that
+        the value of a key is non-empty.