Improve perf. of mod version parsing

Add a SimpleStack model which is identical to the regular Stack model
except for not being a Backbone.Model subclass.  Since so many stacks
are created during mod version parsing, this makes about a 50%
difference in performance during loading.

src/scripts/models/parser_versions/mod_version_parser_v1.coffee

@@ -10,7 +10,7 @@ Item                     = require '../item'
 ItemSlug                 = require '../item_slug'
 ModVersion               = require '../mod_version'
 Recipe                   = require '../recipe'
-Stack                    = require '../stack'
+Stack                    = require '../simple_stack'
 StringBuilder            = require '../string_builder'
 
 ########################################################################################################################

src/scripts/models/simple_stack.coffee

@@ -0,0 +1,22 @@
+###
+Crafting Guide - simple_stack.coffee
+
+Copyright (c) 2014-2015 by Redwood Labs
+All rights reserved.
+###
+
+########################################################################################################################
+
+module.exports = class Stack
+
+    constructor: (attributes={}, options={})->
+        if not attributes.itemSlug? then throw new Error 'attributes.itemSlug is required'
+        attributes.quantity ?= 1
+
+        @itemSlug = attributes.itemSlug
+        @quantity = attributes.quantity
+
+    # Object Overrides #############################################################################
+
+    toString: ->
+        return "#{@quantity} #{@itemSlug}"