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

test/test.coffee

@@ -28,15 +28,16 @@ require '../src/scripts/underscore_mixins'
 
 mocha.setup 'bdd'
 
-require './crafting_plan.test'
+# tests are roughly in order of how errors should be tackled
+require './string_builder.test'
+require './item_slug.test'
 require './inventory.test'
-require './inventory_parser.test'
+require './recipe.test'
+require './mod_version.test'
 require './mod.test'
 require './mod_pack.test'
-require './mod_version.test'
 require './parser_versions/mod_version_parser_v1.test'
-require './recipe.test'
-require './string_builder.test'
+require './crafting_plan.test'
 
 mocha.checkLeaks()
 mocha.globals ['LiveReload']