Consolidate all the random code into a random helper that offers:

random::hash()
  random::string()
  random::percent()
  random::int()

So that we don't have lots of different ways to get random values all
over the code.  Follow-on to #1527.

modules/digibug/tests/Digibug_Controller_Test.php

@@ -36,7 +36,7 @@ class Digibug_Controller_Test extends Gallery_Unit_Test_Case {
     access::deny(identity::registered_users(), "view_full", $album);
 
     $proxy = ORM::factory("digibug_proxy");
-    $proxy->uuid = md5(mt_rand());
+    $proxy->uuid = random::hash();
     $proxy->item_id = $photo->id;
     return $proxy->save();
   }