Make nodes field-translatable

Ready for review -- Last peer review: 11 Sep 2014
Steps: 

Current best thinking is we need to introduce form level language (http://drupal.org/node/1498874 and http://drupal.org/node/1498880)
Once we figure that out, we need to add a translatability setting per node type / entity bundle. (No issue for that yet to try avoid distracting people, ha).