Fixed #2445 -- Allowed limit_choices_to attribute to be a callable.

ForeignKey or ManyToManyField attribute ``limit_choices_to`` can now
be a callable that returns either a ``Q`` object or a dict.

Thanks michael at actrix.gen.nz for the original suggestion.

tests/admin_views/models.py

@@ -173,6 +173,33 @@ class Sketch(models.Model):
         return self.title
 
 
+def today_callable_dict():
+    return {"last_action__gte": datetime.datetime.today()}
+
+
+def today_callable_q():
+    return models.Q(last_action__gte=datetime.datetime.today())
+
+
+@python_2_unicode_compatible
+class Character(models.Model):
+    username = models.CharField(max_length=100)
+    last_action = models.DateTimeField()
+
+    def __str__(self):
+        return self.username
+
+
+@python_2_unicode_compatible
+class StumpJoke(models.Model):
+    variation = models.CharField(max_length=100)
+    most_recently_fooled = models.ForeignKey(Character, limit_choices_to=today_callable_dict, related_name="+")
+    has_fooled_today = models.ManyToManyField(Character, limit_choices_to=today_callable_q, related_name="+")
+
+    def __str__(self):
+        return self.variation
+
+
 class Fabric(models.Model):
     NG_CHOICES = (
         ('Textured', (