Allow prefixes to be specified.

Allows routes to be scoped by admin and user prefixes.

I can remove the change in spec/dummy/config/environment.rb and remove the changes to the specs, if you want, since the routes are already tested in the routing specs.
Line is too long. [87/80]<br>Use the new Ruby 1.9 hash syntax.<br>Prefer double-quoted strings unless you need single quotes to avoid extra backslashes for escaping.