Add :allow_destroy option to has_many forms

This option will add a boolean field "Remove" to the 
end of each `has_many` fieldset that is an existing
record. This provides a workaround for issue #1990, 
and allows for cleaner form coding.

Instead of the following, which would not work because
of the issue mentioned above:

```ruby
form do |f|
  f.inputs do
    f.has_many :children do |child_f|
      child_f.input :name
      child_f.input :_destroy, :as => :boolean, :label => "Remove", :wrapper_html => {:class => 'has_many_remove'} unless child_f.object.new_record?
    end
  end
end
```

we could simply write:

```ruby
form do |f|
  f.inputs do
    f.has_many :children, :allow_destroy => true do |child_f|
      child_f.input :name
    end
  end
end
```