controllers: Add HTTPVerbController

The `HTTPVerbController` base class can be used by controllers to delegate
request handling to different instance methods based on the HTTP request
method.

src/milla/controllers.py

@@ -88,3 +88,42 @@ class FaviconController(Controller):
                    datetime.timedelta(days=self.EXPIRY_DAYS))
         response.headers['Expires'] = milla.util.http_date(expires)
         return response
+
+
+class HTTPVerbController(Controller):
+    '''A controller that delegates requests based on the HTTP method
+
+    Subclasses of this controller should have an instance method for
+    every HTTP method they support. For example, to support the ``GET``
+    and ``POST`` methods, a class might look like this:
+
+    .. code-block:: python
+
+        class MyController(HTTPVerbController):
+
+            def GET(self, request):
+                return 'Hello, world!'
+
+            HEAD = GET
+
+            def POST(self, request):
+                return 'Thanks!'
+
+    This example also allows ``HEAD`` requests, by processing them as
+    ``GET`` requests. *Milla* handles this correctly, as it does not
+    send a response body for ``HEAD`` requests, even if the controller
+    callable returns one.
+    '''
+
+    def __call__(self, request, *args, **kwargs):
+        try:
+            func = getattr(self, request.method)
+        except AttributeError:
+            raise milla.HTTPMethodNotAllowed
+        return func(request, *args, **kwargs)
+
+    @property
+    def allowed_methods(self):
+        for attr in dir(self):
+            if attr.upper() == attr:
+                yield attr