created sum method

lib/underscore.lua

@@ -104,6 +104,10 @@ function Underscore.funcs.reduce(list, memo, func)
 	return memo
 end
 
+function Underscore.funcs.sum(list) 
+	return Underscore.funcs.reduce(list, 0, function(a, b) return a + b end) 
+end
+
 function Underscore.funcs.detect(list, func)
 	for i in Underscore.iter(list) do
 		if func(i) then return i end

spec/sum_spec.lua

@@ -0,0 +1,11 @@
+require 'spec_helper'
+
+describe["_.sum"] = function()
+  it["should return the sum of all elements of the list"] = function()
+    input = { 1,2,3 }
+    result = _.sum(input)
+    expect(result).should_be(6)
+  end
+end
+
+spec:report(true)