Changed banner description to text for advanced integration of images, e...

...tc
Yes, I hope to keep compatibility because some users who already installed this plugin have to change column type manually.  So, I think add file to migrate like this:

005_change_column_banner_description
```
class ChangeColumnBannerDescription < ActiveRecord::Migration
  def self.up
    change_column :banners, :banner_description, :text
  end

  def down
     change_column :banners, :banner_description, :string
  end
end  
```