[BUGFIX release] Update Backburner.js

Includes improved debug performance
Merge pull request #4011 from ebryn/update-backburner

[BUGFIX release] Update Backburner.js
[DOC beta] Cleanup Ember.TextField docs.
Merge pull request #4012 from rjackson/cleanup_text_field_docs

[DOC beta] Cleanup Ember.TextField docs.