Rails 4 compatibility

I'm working to make SimpleForm compatible with Rails 4.

## Release plan

We choose the drop support to Rails 3 since SimpleForm 2.x seems stable for those who want to use it with Rails 3 applications.

We are planing to release two new versions:

* **SimpleForm 3** supporting only Rails 4
* **SimpleForm 2.1** with the same set of feature of SimpleForm 3, but supporting only Rails 3

## Dependency

We need to make `country_select` compatible with Rails 4 first. There is a pull request open to it. https://github.com/stefanpenner/country_select/pull/7