Added call stack example

examples/multi-callstack.js

@@ -0,0 +1,30 @@
+
+/* Multi Call Stack */
+
+var Multi = require('../');
+
+var context = {
+  sum: function(a,b,cb) {
+    cb(null, a+b);
+  },
+  hello: function(name, cb) {
+    cb(null, 'Hello ' + name + '!');
+  }
+}
+
+var multi = new Multi(context, {flush: false});
+
+multi.sum(1,2);
+multi.sum(3,4);
+multi.hello('Ernie');
+
+// Run `exec` for the first time
+multi.exec(function(err, results) {
+  console.log(err || results);
+});
+
+// Any upcoming `exec` calls will still
+// make use of the original call stack
+multi.exec(function(err, results) {
+  console.log(err || results);
+});