More specific conditional for Node detection

Hi, I have been using your library with the Thorax Backbone framework, and last week they updated their package structure to include Backbone using Bower. 

Since Thorax has a module 'exports' object but no 'require' function, it would trip the Node conditional on initialize, but fail because the require object doesn't exist. I just made it a little more specific, and it fixed my problem by attaching to the window Backbone object instead of trying to require it in. I hope it jives with you.

Chris