Added UMD wrapper

src/backbonefire.js

@@ -8,7 +8,7 @@
  * License: MIT
  */
 
-(function(_, Backbone) {
+var backbonefire = function(_, Backbone) {
   'use strict';
 
   Backbone.Firebase = {};
@@ -864,4 +864,27 @@
 
   });
 
-})(window._, window.Backbone);
+};
+
+/*globals self, global, define, require, module */
+(function(backbonefire) {
+  'use strict';
+  var root = (typeof self == 'object' && self.self == self && self) ||
+    (typeof global == 'object' && global.global == global && global);
+
+  if (typeof define === 'function' && define.amd) {
+    // AMD. Register as an anonymous module.
+    define(['underscore', 'backbone'], function(_, Backbone) {
+      backbonefire(_, Backbone);
+    });
+  } else if (typeof module === 'object' && module.exports) {
+    // Node/CommonJS
+    var _ = require('underscore');
+    var Backbone = require('backbone');
+    module.exports = backbonefire(_, Backbone);
+  } else {
+    // Browser globals
+    backbonefire(root._, root.Backbone);
+  }
+
+})(backbonefire);