Core Twitter embed card fixes:

- Fixed twttr undefined error. 
- Made sure embeds still work when another 
Twitter widget is included in page (e.g., in a 
pocket). 
- Embed cards will now immediately display when
posted, instead of on page refresh.
- No timeout hack to ensure load (now using promises).
- Will not break CLEditor and Advanced Editor.
- Embed cards will transition in. 
- Refactored code.

See http://vanillaforums.org/discussion/26739/codestyle for example of twttr undefined, which consequently breaks the editor.