Add support for Foundation 5

Olà !

(related to #1090)

I'm a big fan of **Simple form**, but also I start enjoying building apps with **Foundation 5** and I was a bit sad of not having a recent way to build nice foundation simple_forms.

Thus, I took your excellent [Simple_form-bootstrap](http://simple-form-bootstrap.plataformatec.com.br) application and forked it in order to make it work with Foundation 5.
The demo is [here](https://github.com/jorge-d/simple_form-foundation).

I also have some screenshots for you:
![screen shot 2014-07-29 at 21 08 40](https://cloud.githubusercontent.com/assets/1178099/3740304/f30c94b6-1753-11e4-89ab-1a9221a0a494.png)

![image](https://cloud.githubusercontent.com/assets/1178099/3740288/d4559bda-1753-11e4-9dfd-a03a93b00430.png)

So this PR changes the existing initializer and makes it handle pretty much all cases. If you have any question, or feedback I'll be glad to answer it,

Muito obrigado,
This comments were added as documentation at https://github.com/jorge-d/simple_form/commit/ff5435c645b9da1fc774676d85b5eafe5240b203