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,
:+1: