Replace guardian witness logo by text

- [New] Witness logo is plain text
- [New] Whole witness block is clickable (previously, only the text and the + were clickable)
- [Fixed] Buggy SVGs that contained weird meta headers

The witness SVG logo won't render when sprite generation happens.
So I changed it into text. It's not exactly the same font weight but it's more accessible and a lot more robust.

It saves us a few KB (about 5 to 10 KB, not sure: svg + png sprites) on first page load.

Fixes #686 and #1612.

## Before

### PNG fallback

![screen shot 2013-09-25 at 16 42 59](https://f.cloud.github.com/assets/85783/1210400/2d12ef86-25f9-11e3-9b07-e80b20d5869e.PNG)


### SVG on

![screen shot 2013-09-25 at 16 41 58](https://f.cloud.github.com/assets/85783/1210394/194a1d6c-25f9-11e3-83bb-0003d7f1d40a.PNG)


## After

![screen shot 2013-09-25 at 16 39 19](https://f.cloud.github.com/assets/85783/1210395/20cbc612-25f9-11e3-8a92-3dbbaaa9d035.PNG)


![ ](http://s1.developerslife.ru/public/images/gifs/9e8e0f99-7c78-478b-8fb0-c56d75e57ecd.gif)