def sortable(att, value)
      sortables[id][att].set(value)

      fields[SORTABLES].sadd att
    end
def sortable(att, value)
      sortables[id][att].set(value)

      fields[SORTABLES].sadd att
    end
Adds a sortable index for `att` with `value`.

 @example

   class Gadget
     def self.[](id)
       # find the gadget using id here
     end
   end

   Lunar.index Gadget do |i|
     i.id 1001
     i.text 'apple macbook pro'
     i.sortable :votes, 50
   end

   Lunar.index Gadget do |i|
     i.id 1002
     i.text 'apple iphone 3g'
     i.sortable :votes, 20
   end

   results = Lunar.search(Gadget, :q => 'apple')
   results.sort(:by => :votes, :order => 'DESC')
   # returns [Gadget[1001], Gadget[1002]]

   results.sort(:by => :votes, :order => 'ASC')
   # returns [Gadget[1002], Gadget[1001]]

 @param [Symbol] att the field you want to have sortability.
 @param [String, Numeric] value the value of the sortable field.

 @return [String] the response from the redis server.