admin: use {$ne:null} instead of {$exists:true} for listing contributors

in admin. Who knew $exists had such poor performance!
https://groups.google.com/forum/#!topic/mongodb-user/FKJWoi3lb9I
@sabrecat @wc8, admin page should be snappier now

src/controllers/admin.js

@@ -23,7 +23,7 @@ api.getMember = function(req, res) {
 }
 
 api.listMembers = function(req, res) {
-  User.find({'contributor.level':{$exists:true}})
+  User.find({'contributor.level':{$ne:null}})// {$exists:true} causes terrible performance http://goo.gl/GCxzC9
     .select('contributor balance profile.name')
     .sort('contributor.text')
     .exec(function(err, users){