Retina #brand logo in the navbar

I replaced the `e_small.png` and `e_red_small.png` images with the same images but with their sizes doubled. This way the logo looks nice on retina screens. I also had to modify some CSS in order to constrain the images (which are just `background`s) into a fixed size, since the previous CSS relied only on the actual size of the images.

Before:
![screen shot 2014-11-25 at 23 10 04](https://cloud.githubusercontent.com/assets/3890250/5192499/526eb3b0-74f8-11e4-818b-f9ae8befab3b.png)

After:
![screen shot 2014-11-25 at 23 10 13](https://cloud.githubusercontent.com/assets/3890250/5192503/5577e446-74f8-11e4-9c1a-e98c63a99968.png)

The logo is *slighlty* different (well, I'm no designer) since I couldn't find that small brand icon in the Illustrator files that @kytrinyx provided me. I'll let the collaborators decide if it's still as good.

Thanks!

**PS** Obviously the hovered red version of the image has been updated too :smiley:.