Move form_for helper into core_ext/rails

The `form_for` helper is currently implemented as an override of the `form_for`
method provided by ActionView, and therefore should only be available in Rails.
Move link_to helper into core_ext/rails

The `link_to` helper is currently implemented as an override of the `link_to`
method provided by ActionView, and therefore should only be available in Rails.
Merge branch 'move-into-core-ext'
Rename internal module AlertHelper->AlertBoxHelper
Rename alert_box.rb to alert_box_helper.rb
Merge branch 'rename-alert-box-helper-module'
Move alert_box helper spec into spec/shared
Pass `alert_box` extra params to the alert <div>

Before this commit, the `alert_box` helper ignored unknown options.
For instance the code:

```rhtml
<%= alert_box message, priority: priority, class: 'important', id: 'alert' %>
```

would simply generate the following HTML

```html
<div class="alert alert-success" role="alert">
  ...
</div>
```

ignoring the `:class` and `:id` parameter. After this commit, the result is:

```html
<div class="important alert alert-success" id="alert" priority="notice" role="alert">
  ...
</div>
```
Merge pull request #42 from claudiob/pass-extra-class-in-alert-box

Pass extra class in alert box