have Urls generate paths given named/ordered params

This adds behavior so that a Url can generate a path for given
parameter data.  Params can be name (given as a Hash) or ordered
and will be applied to the source path appropriately.

This also adds a convenience `url` method on the server for looking
up a named url and generating a path given some params data.
Merge pull request #85 from redding/kr-urlmeth

have Urls generate paths given named/ordered params