Removed use of `memorize` in EmberApp. Allows multiple EmberApps to be instantiated.

See #1361 for details.

This feels a bit like a lazy commit. I really had no idea how to test a broccoli tree and it seems there isn't much of that yet. I thought about testing subsequent calls to `this._processedVendorTree` to compare objects ... but that's really testing a private API. 

I'd love to get some tests and I'm open to any suggestions.
Happy to.