Print app url at startup

This uses the `Path.build_url` to build the url. Defaults to 'localhost' if `options[:host]` is unset.

The port isn't an argument to `Path.build_url`, so I've left that as is for now. It would be good if it were possible to pass in port & build a full url. (but that's probably a separate issue)

(related to issue #129)
I tried that, but since it's mapping over a list of keys, if `:host` isn't in the list, that clause will never get hit. :/