Use defeatureify to strip debug statements from output.

* Adds `grunt-ember-defeatureify`
* Adds `dist` task, which now is roughly equivalent to previous `rake dist`.
* Removes `uglify` from `test` task (moved to `dist`).
Merge pull request #1702 from rjackson/use-defeatureify-to-strip-debug-statements

Use defeatureify to strip debug statements from output.