Check for global Ember in Node

After the module.exports check (which presumably checks whether the architecture is Node-like), there is an implicit assumption that the 'ember' module is available. However, since 'ember' is not listed in the dependencies within package.json, this is a faulty assumption. In fact, including it in package.json would probably be a pretty bad idea anyway, since the Ember npm module hasn't been updated in a year. I suspect most people using Ember and Node (or in my case, Node + Browserify) are running Ember by directly requiring it from a local file, and would therefore have ```Ember``` in global scope, while a ```require('ember')``` would fail