adding title for forms

For #2195

Usage : 
Example 1:
```ruby
ActiveAdmin.register Category do
  form do |f|
    f.inputs do
      f.input :name
      f.input :description
    end
    f.actions
  end
end
```

Edit and New Forms shows Default Titles (Edit Category and New Category)

Example 2:
```ruby
ActiveAdmin.register Category do
  form :title => "A new title" do |f|
    f.inputs do
      f.input :name
      f.input :description
    end
    f.actions do
      f.action :submit
      f.action :cancel, :label => 'Cancel', :input_html => {:class => 'cancel_subscription'}
    end
  end
end
```
Edit and New Forms shows the new Title ("A new title")


Example 3:
```ruby
ActiveAdmin.register Category do
  form :title => { :new => "New Category", :edit => "Renew  Category"} do |f|
    f.inputs do
      f.input :name
      f.input :description
    end
    f.actions do
      f.action :submit
      f.action :cancel, :label => 'Cancel', :input_html => {:class => 'cancel_subscription'}
    end
  end
end
```
Edit and New Forms shows the new titles ("Renew Category" and "New Category" respectively)