version to 0.19.0

* rename `url` to `url_for`; add `url` method for defining named urls(#90)
* allow specifying a url name as the "from path" on a redirect (#91)
* allow specifying a url name as the "from path" on a route (#92)
* some cleanups to the routing API (#93)
* move all server routing concerns into a `Router` object (#94)