[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.

See also:
* https://github.com/emberjs/ember-dev/pull/118
* https://github.com/thomasboyt/defeatureify/pull/6
* https://github.com/thomasboyt/defeatureify/pull/8

Hard work done by @thomasboyt & @pangratz.
[DOC beta] Wrap long Ember.assert messages.