Json render

Follow of #87 :

I think I'm done. I face a problem where the view is not found :

```
ActionView::MissingTemplate (Missing template devise_token_auth/registrations/create_errors with {:locale=>[:en], :formats=>[:html], :variants=>[], :handlers=>[:erb, :builder, :raw, :ruby, :jbuilder, :rabl]}.
```

We can see that the formats of the request if `:html`. I have to force my route to display JSON :

``` Ruby 
constraints format: 'json' do
   mount_devise_token_auth_for 'User', at: '/users/auth'
end

```