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/mod.coffee

@@ -87,13 +87,13 @@ module.exports = class Mod extends BaseModel
         return unless @_activeModVersion?
         @_activeModVersion.findItemByName name
 
-    findName: (slug)->
+    findName: (itemSlug)->
         return unless @_activeModVersion?
-        @_activeModVersion.findName slug
+        @_activeModVersion.findName itemSlug
 
-    findRecipes: (slug, result=[])->
+    findRecipes: (itemSlug, result=[])->
         return result unless @_activeModVersion?
-        @_activeModVersion.findRecipes slug, result
+        @_activeModVersion.findRecipes itemSlug, result
 
     # Property Methods #############################################################################
 
@@ -123,6 +123,7 @@ module.exports = class Mod extends BaseModel
 
         for modVersion in @_modVersions
             return modVersion if modVersion.version is version
+
         return null
 
     getActiveVersion: ->
@@ -150,7 +151,6 @@ module.exports = class Mod extends BaseModel
             @trigger Event.change + ':activeVersion', this, @_activeVersion
             @trigger Event.change, this
 
-
     # Backbone.View Overrides ######################################################################
 
     parse: (text)->