Replace MiddlemanView with PadrinoView

The only relevant difference between the two is that Middleman
overrides Padrino’s `url_for` to use `app.req` rather than
`app.request`.
Rename spec/middleman to spec/padrino
Remove duplicate code for `link_to`
Merge pull request #101 from claudiob/remove-duplicate-link-to

Remove duplicate link to