[beta] Use defeatureify to strip debug statements.

Recent changes to defeatureify allow stripping of debug messages. This will
allow us to use multi-line debug statements.

To enable this the format of features.json has changed.

An example illustrating that it is now possible to strip multi-line `Ember.assert` is included as a separate commit.

See also:
* https://github.com/emberjs/ember-dev/pull/118
* https://github.com/thomasboyt/defeatureify/pull/8