Adds editor, introducer and translator to accessible attributes.

This is mostly to allow books to be edited through RailsAdmin. This
would be better handled using roles, allowing access only to admin and
daemon roles.

app/models/source.rb

@@ -2,7 +2,7 @@ class Source < ActiveRecord::Base
 
   include SyncHelper
 
-  attr_accessible :title, :author, :editor, :lang, :published_date, :pages, :isbn_10, 
+  attr_accessible :title, :author, :editor, :introducer, :translator, :lang, :published_date, :pages, :isbn_10,
   :isbn_13, :page_count, :google_books_id, :publisher, :library_thing_id, :open_library_id,
   :tag, :dirty, :attempts, :notes