Updated assets to Tango guidelines.

New corebird icon based on the original. 
![corebird_48](https://f.cloud.github.com/assets/4263475/1506474/5831bae8-492c-11e3-9e68-36d4c227ee20.png)
New alternative corebird icon.
![corebird_alternative](https://f.cloud.github.com/assets/4263475/1506801/7b37f372-494a-11e3-914c-09707549b84a.png)
Added pixel perfekt verified image.
Tangofied verified image.
![verified](https://f.cloud.github.com/assets/4263475/1506473/53ec182a-492c-11e3-942c-478dc31df5bd.png)
Added no_banner.svg for easy adjustment of the gradient.
Tangofied no_banner image.
![no_banner](https://f.cloud.github.com/assets/4263475/1506472/53d755d4-492c-11e3-924f-641234142dbb.png)
Added pixel perfect no_avatar image.
Tangofied no_avatar image.
![no_avatar](https://f.cloud.github.com/assets/4263475/1506471/53b11d24-492c-11e3-9782-304382a4d271.png)
Removed render-images.sh.
Added render.sh with more functionality.

All done in mind with the tango icon guidelines:
http://tango.freedesktop.org/Tango_Icon_Theme_Guidelines