Presence object to s

```see```, ```not_see``` and ```click``` can also work with objects, that respond to overwritten to_s method. Example: You have list of users and you want navigate to someone's profile

Model:
``` ruby
class User < ActiveRecord::Base
  def to_s
    full_name
  end

  def full_name
    "#{first_name} #{last_name}"
  end
end
```

HTML:
``` html
<ul>
  <li><a href="/profile/mc"/>Michal Czyz</a></li>
  <li><a href="/profile/tb"/>Tomasz Bak</a></li>
</ul>
```

Spec:
``` ruby
user = User.create(first_name: 'Michal', last_name: 'Czyz')
...
see user
click user
```