Params: rewrite get_number() to align to new Router API

This patch reimplement the param->get_number() method where now it
uses the new Router mechanism to retrieve numeric values coming from
the URL, e.g:

  Router:  /test/:name/:id/about/
  URL   :  /test/eduardo/12345/about/

when using the API:

  int ret;
  long val

  ret = param->get_number(dr, "name", &val);
  if (ret == 0) {
     /* val = 12345 */
  }

Signed-off-by: Eduardo Silva <eduardo@monkey.io>