Check for global Ember in Node

After the module.exports check (which presumably checks whether the architecture is Node), there is an implicit assumption that the 'ember' module is available. However, since 'ember' is not listed in the 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, but no ember module to require.