Expand links with Embed.ly

Added support for expanding links/images with [Embed.ly cards](http://embed.ly/cards).

Effectively reverts #228 and removes the `prefetch`, `links` and `thumbnails` options (plus a bunch of dependencies).

In action:-

![screen](http://i.imgur.com/p662AvW.png)

Settings:-

![settings](http://i.imgur.com/dKqofrf.png)