Added support for embeddable tweets

Added support for embeddable tweets

You can easily embed any tweet to your posts by simply copy pasting their full url in markdown
E.g. 
https://twitter.com/xdamman/status/408455694241636352

The only issue is that because of the live update of the preview pane, the embeddable tweet needs to be re-rendered every time. This creates a flickering which is not ideal. I mitigated it by setting a min height to the container of the tweet. Not sure how we can avoid that. I'm not familiar enough with your code base yet. Please have a look.

I'm happy to add support for other types of embeds (youtube, instagram, facebook, ...) if there is interest. 

![screenshot](https://photos-3.dropbox.com/t/0/AAAKYCE5MfAzvxt1AwNL137DlCRKDoZUdQBMKGrT6Zl8zw/12/1702667/png/1024x768/3/1386241200/0/2/Screenshot%202013-12-05%2001.15.23.png/s071uGo_mproSc0NcomVtHvY7NvgbUJ5tNwRxnGJYQ0)