remove app.router and refactor middleware processing

This is an overhaul of middleware processing, Router and Route. Connect is no
longer used to process the middleware stack. This functionality has been
split into two parts: middleware stack and default error response.

The entry point for request processing is the `app.handle` method. It
sets up the default error response handle (to run in the event of no
other error handler) and then triggers the app router (instance of
Router) to handle the request.

The app router `handle` function contains the middleware dispatch layer
previously in the connect codebase. This layer handle the logic for
dispatching `.use` calls (stripping paths if needed). The app contains a
base router `app._router`. New routes can be created and `.use`d on this
router to organize routes into files.

Routers now have the following methods `.use`, `.all`, `.param` which
are all public.

Additionally, Routers have a `.route(path)` method which returns a new
instance of Route for the requested path. Route(s) are isolated
middleware stacks and contain methods for the HTTP verbs as well as an
`.all` method to act similar to middleware. These methods are chainable
to easily describe requirements for a route.

  var route = Router.route('/foo'); // or 'app.route('/foo')'

  route
  .all(auth)
  .get(function(...) {})
  .all(more_checks)
  .post(function(...) {})

Any Route and Router methods which accept handlers also accept error
(arity 4) handlers which will also behave as expected.

Finally, the `app.router` getter has been removed. Middleware and
handlers are run IN THE ORDER they are seen in the file. This means that
code which injected the `app.router` and then added error handlers (or
other middleware) will need to be updated to move those handlers after
any requests added on the app object. The examples have been updated
accordingly. This is the largest breaking change to codebases in this
commit.