Defeatureify

Per recent conversations in #ember-dev, this adds support for feature conditionals as described [here](https://gist.github.com/thomasboyt/a5f4e04e83365eb40afd).

It relies on the `defeatureify` NPM package, so to build a non-Canary build, you'll need to install it:

```
npm install -g defeatureify
```

(travis builds are dependent on #3322. Also builds appear to be breaking when using ember-dev from Git instead of a local directory, not sure what's going on there.)