Burger icon

Small refactor of the `all sections` burger icon. Using pseudo elements now instead of three icons. I've tested it using VM and Browserstack and it seems to be working without any issues everywhere. I did some small cleaning on the way.

Before:
![all-old](https://cloud.githubusercontent.com/assets/2579465/4939758/445f71e2-65d5-11e4-8572-961ec5519d21.png)

After:
![all-new](https://cloud.githubusercontent.com/assets/2579465/4939766/4c5de4e6-65d5-11e4-9701-47b2b2db280f.png)

Paddings are a little bit cleaner now.

On IE8- it will fallback just to text:
![screen shot 2014-11-06 at 14 38 15](https://cloud.githubusercontent.com/assets/2579465/4939786/66d49b30-65d5-11e4-88de-4f2a064e5e1c.png)

The burger icon size is 12px wide as default and from tablet it goes to 16px. Do we really need this? @paulrobertlloyd @sammorrisdesign as it makes almost no difference.

CC @mattosborn as you were asking about this before