add is null filter, add complex fiters (and_ & or_) and basic tests for them

docs/queries/filter-and-sort.md

@@ -70,6 +70,8 @@ You can use special filter suffix to change the filter operands:
 * contains - like `album__name__contains='Mal'` (sql like)
 * icontains - like `album__name__icontains='mal'` (sql like case insensitive)
 * in - like `album__name__in=['Malibu', 'Barclay']` (sql in)
+*  isnull - like `album__name__isnull=True` (sql is null)
+   (isnotnull `album__name__isnull=False` (sql is not null))
 * gt - like `position__gt=3` (sql >)
 * gte - like `position__gte=3` (sql >=)
 * lt - like `position__lt=3` (sql <)

docs/releases.md

@@ -1,3 +1,13 @@
+# 0.9.7
+
+## Features
+* Add `isnull` operator to filter and exclude methods. 
+    ```python
+    album__name__isnull=True #(sql: album.name is null)
+    album__name__isnull=False #(sql: album.name is not null))
+    ```
+
+
 # 0.9.6
 
 ##Important