Fix the inline help being invisible with addons

when using the `:add_on => :prepend` option, the help-inline becomes invisible.

the following code will reproduce the issue when submitting the following form with an invalid value:
```
= twitter_bootstrap_form_for(@user, :html => { :class => "form-horizontal" }) do |user|
    = user.text_field :twitter_id, :add_on => :prepend do
      %span.add-on @
```

this is because the help-inline span wants to be at the same level as the input-prepend div, but it is within it. Attached code puts it in the correct place.