Add suport for Response schemas (#10)

Schemas generated from Serializers will now be added to the `definitions` section by default, and used as `$ref` objects where needed.  
The Schema definition name is based on the serializer class name, and can be overriden by specifying a `__ref_name__` property on the Serializer. If this property is set to None, the schema will not be added to `definitions` and will be forced inline.

Closes #6, #7.

requirements/test.txt

@@ -2,6 +2,7 @@
 pytest-django>=3.1.2
 pytest-pythonpath>=0.7.1
 pytest-cov>=2.5.1
+datadiff==2.0.0
 
 # test project requirements
 Pillow>=4.3.0