update ember-cli addon implementation

I've simplified the ember-cli addon file to be more "modern". 

Main differences:

* exports an object instead of function
* removed requiring of broccoli-merge-trees and broccoli-static-compiler
* set up for a clean upgrade to using `treeForVendor` once https://github.com/stefanpenner/ember-cli/pull/1930 is in the stable release.