Fallback when current-resource return current file

As mentioned in #22, when the current file is a model, if the user calls
`projectile-rails-find-current-model`, we should list all the
models. Currently nothing will be done.

Now, with the feature described in #22 implemented, key-chord users can
define suck key bindings:

```el
(key-chord-define 'projectile-rails-mode-map "jm" 'projectile-rails-find-current-model)
(key-chord-define 'projectile-rails-mode-map "jc" 'projectile-rails-find-current-controller)
(key-chord-define 'projectile-rails-mode-map "jv" 'projectile-rails-find-current-view)
(key-chord-define 'projectile-rails-mode-map "jh" 'projectile-rails-find-current-helper)
```

This makes it extremely fast to toggle among current resources.