Add URL module for translating URLs
Fix all the specs that were broken by Domain change
Alter behavior of Doc::URL.

It is now a class that takes a domain and returns
nil if a requested URL is not supported by the domain.
Update Racks and specs for new URL implementation
Greatly simplify Rack stack (this breaks many tests)
Rewrite application_spec as rack_spec, get it passing.
Rewrite stylesheet_spec as css_spec
Write Rack::HTML spec.
Add a Rack::Subject spec
Add Rack::URL spec
Fix up / remove remaining specs
Touch up some doc