improved compatibility with node and node console

[jcollum]

All tests are passing.

Usage:

und = require('underscore'); 
observableMixin =  require('./assets/js/lib/underscore.observable.js'); 
observableMixin(und);

Maybe it should be taking a guess at the existence of the _ variable when it's loaded via require?

[asvarga]

Hi, I'm having trouble getting this to work with node. When I use the above code with the appropriate reference to underscore.observable.js, I get an error at line 196: "_.mixin({ ..." saying "ReferenceError: _ is not defined". What might I be doing wrong? Thanks.