Add benchmark for the router.

This is for future performances' regressions.
Rename a few files.

The dispatcher is renamed router (as for the test file) and the
router_test is renamed to route_test (since that's what this file is
supposed to test).
Rename all instances of Dispatcher to Router.
Add exemple for validations in the README.
Support default value for placeholders in route's path.

If a route is created with a Default:

   &Route{
        Path: "/user/:id",
        Defaults: map[string]string{
                  "id": "1",
        },
        ...
   }

a request for /user/ will match the route, and set the value for "id" to
1.  However, the route /user will not match.