add features to show how namespaced routes can be matched

[jredville]

Document how to use namespaced routes in route specs.

[travisbot]

This pull request passes (merged e795ec0 into f869484).

[justinko]

Thanks for the pull!

I ended up reverting[1] the changes to the README, since the documentation[2] for #route_to is sufficient (it advises to reference assert_recognizes).

1.) 9abdc0a
2.) https://github.com/rspec/rspec-rails/blob/master/lib/rspec/rails/matchers/routing_matchers.rb#L44-59