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
- [Fixed] the "plus" icon bleeds out 1px on the bottom of the block (used to bleed out at the top only)

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)