Improved Branching support for Node/CommonJS, AMD and Browser globals

[benbabics]

I forked this while using in-conjunction with RequireJS, so I thought I'd issue a pull request. This is similar to the branching found for the BackboneJS AMD support.

[PaulUithol]

Duplicate of #57. Following Backbone's lead on this one..

[benbabics]

Perhaps you're using a newer version of Backbone with AMD support? I've followed and modeled my change after the AMD branch:
https://raw.github.com/amdjs/backbone/master/backbone.js

This approach may work for Node/CommonJS, but the following breaks when using AMD as it has a dependency on Backbone (just as Backbone does for Underscore):

require([ 'underscore', 'backbone', 'backbone-relational' ], function( _, Backbone ) {
  // 
});

This will throw an error because Backbone is undefined when using AMD. The approach Backbone AMD (which I followed) takes accounts for AMD support.

[hacking-robot]

AMD fork: https://github.com/dexnode/Backbone-relational/blob/master/backbone-relational.js