Convert codebase to using ItemSlug

* Flesh out the formerly incomplete ItemSlug class
* Convert all uses of text item slugs to use ItemSlug
* Move the ItemSlug class under models where it properly belongs
* Avoid the use of `slug` as a local variable or item attribute
  wherever possible to avoid confusion between various kinds of slugs
* Merge the InventoryParser class into the Inventory class as it is
  unlikely to need to support multiple versions
* Simplify the ModPack/Mod/ModVersion interfaces by replacing the use
  of various "helper" methods with equivalent use of more generic
  methods
* Update the various methods of ModPack/Mod/ModVersion to be able to
  work with qualified or unqualified ItemSlugs
* Remove the unused BaseCollection class
* Add tests for ItemSlug and update all other tests to reflect
  all the other changes

src/scripts/models/stack.coffee

@@ -12,7 +12,7 @@ BaseModel = require './base_model'
 module.exports = class Stack extends BaseModel
 
     constructor: (attributes={}, options={})->
-        if not attributes.slug? then throw new Error 'attributes.slug is required'
+        if not attributes.itemSlug? then throw new Error 'attributes.itemSlug is required'
         attributes.quantity ?= 1
         options.logEvents   ?= false
         super attributes, options
@@ -20,4 +20,4 @@ module.exports = class Stack extends BaseModel
     # Object Overrides #############################################################################
 
     toString: ->
-        return "#{@quantity} #{@slug}"
+        return "#{@quantity} #{@itemSlug}"