Task
Use Twig instead of PHPTemplate
Twig sandbox: http://drupal.org/sandbox/pixelmord/1750250
We already have datetime.html.twig but we still need to remove theme_datetime() and likely do a few tweaks to template_preprocess_datetime() and datetime.html.twig while we're at it.
Remaining
- Replace all theme functions and templates with .html.twig equivalent templates
- Add new preprocess functions for the .html.twig equivalent templates
- Update all hook_theme definitions