Try importing rest_framework module (#568)

* Try importing `rest_framework` module

Fixes #567

* Correct Typo

Thanks @kevin-brown!

Author: Carlton Gibson

django_filters/__init__.py

@@ -4,6 +4,13 @@
 from .filterset import FilterSet
 from .filters import *
 
+# We make the `rest_framework` module available without an additional import.
+#   If DRF is not installed we simply set None.
+try:
+    from . import rest_framework
+except ImportError:
+    rest_framework = None
+
 __version__ = '1.0.0'