Pull in extra meta data for links

This PR pulls in extra metadata from links to be used in the inline toggle boxes.

The first commit extends what was already existing and actually gets a description for the site from `<meta property="og:description">` and `<meta name="description">` tags. An example of this follows where the *The official website for the Shout IRC client.* comes from the `<meta>` tag:
![image](https://cloud.githubusercontent.com/assets/7449105/4604692/94024f04-51b1-11e4-8374-7e393a994ba6.png)

The second commit adds a small thumbnail that is gets for the site from `<meta property="og:image">` and `<meta property="twitter:image:source">` tags. An example of this follows where:
![image](https://cloud.githubusercontent.com/assets/7449105/4604697/de1d0912-51b1-11e4-8691-3221460f3a31.png)

![image](https://cloud.githubusercontent.com/assets/7449105/4604698/0055c3ca-51b2-11e4-9695-b25425121086.png)

I haven't added a setting to enable/disable image previews, but it might be a good idea to add one!