create sum() finder example

examples/Sum/app.js

+var Sequelize = require(__dirname + "/../../index")
+  , config    = require(__dirname + "/../../spec/config/config")
+  , sequelize = new Sequelize(config.database, config.username, config.password, {logging: false})
+
+var Person  = sequelize.define('Person',
+  { name: Sequelize.STRING,
+    age : Sequelize.INTEGER
+
+  })
+  , chainer = new Sequelize.Utils.QueryChainer
+
+sequelize.sync({force: true}).on('success', function() {
+  var count   = 10,
+      queries = []
+
+  for(var i = 0; i < count; i++)
+    chainer.add(Person.create({name: 'someone' + (i % 3), age : i+5}))
+
+  console.log("Begin to save " + count + " items!")
+
+  chainer.run().on('success', function() {
+    console.log("finished")
+    Person.sum('age').on('success', function(sum) {
+      console.log("Sum of all people's ages: " + sum)
+    });
+  })
+})