Adds support for nullable url parameters.
(Much) better integrated with MVC4 Controllers

Now invocation of actions respects filters, so that authentication can
work properly.
Basically we now use the MVC ControllerActionInvoker, but override it
to parse parameters and call the action ourselves.
Makes it almost practical to use (though form posts are still a problem).
Adds equality interface to PotentialUrl
Rearranges link generation a bit.

Moves API for creating a URL from a route from HtmlHelper to UrlHelper,
to better match existing MVC4 API.