Update Sass logo

I used the background from the official @Sass website and put the logo on top of it.

### Before

![screen shot 2014-05-13 at 23 11 30](https://cloud.githubusercontent.com/assets/85783/2964864/aa1790de-daeb-11e3-8942-7748a8a66302.PNG)


### After

![screen shot 2014-05-13 at 23 11 21](https://cloud.githubusercontent.com/assets/85783/2964865/aecb896e-daeb-11e3-9ac4-c3feaf3b53f9.PNG)

![such-logo](https://cloud.githubusercontent.com/assets/85783/2964900/2f6feff6-daec-11e3-9426-c031c3cf45fe.gif)