FEATURE: allow full twitter embed

Example:

![screen shot 2014-08-28 at 19 45 38](https://cloud.githubusercontent.com/assets/5732281/4077158/8f88b4ea-2ec0-11e4-990b-47d5d2808cca.png)

*Note: One advantage of using this strategy will be that Twitter API Key will no longer be required for Twitter Oneboxing!*

cc @SamSaffron @coding-horror