asset_path method to be used in sass files

sass_initializer.rb

# Usage
#   body {
#     background: url(asset_path('background.png')) #FFFFFF;
#   }
module Sass::Script::Functions
  include Sprockets::Helpers::RailsHelper
  def asset_path(path)
    assert_type path, :String
    Sass::Script::String.new(super(path.value), :string)
  end
  declare :asset_path, :args => [:string]
end