Support for btn input addons

Adds support for http://getbootstrap.com/components/#input-groups-buttons
Trying this PR locally, I found a couple of things:

1. The `button_to` helper I wrote is incomplete. It only works in the form where the second argument is a route, but it does not work if the second argument is options. This is similar to what happened with `fields_for`. So I need to fix that separately
2. Even though the following code works, the `button_to` helper from ActionView adds an extra `<div>` around the `<input type="button">` field which makes the suffix display in a weird way:

```rhtml
<%= f.text_field :email, suffix: button_to('Go', signin_path) %>
```

With the current master displays as:
![screen shot 2014-09-15 at 10 58 50 am](https://cloud.githubusercontent.com/assets/7408595/4276074/2596ebce-3d02-11e4-8fc1-f9bd9f863fd7.png)

After the current PR would display as:
![screen shot 2014-09-15 at 10 59 20 am](https://cloud.githubusercontent.com/assets/7408595/4276070/23325e0e-3d02-11e4-8bbc-3b502cbd98a0.png)

Which a little different from the desired behavior:

![screen shot 2014-09-15 at 10 59 27 am](https://cloud.githubusercontent.com/assets/7408595/4276069/2183976c-3d02-11e4-9153-51a038ff5337.png)