Rename main.js to ember-model.js

This change makes ember-model play nicer with the Rails asset pipeline when used with bower: bower checks out the git repo into the rails project in a subdirectory called bower_components, which sprockets can traverse to look for a dependency. Before this change, one needs to put `//= require ember-model/lib/main` in application.js; after this change, `//= require ember-model` will work as well.