Update icon

With Yosemite it could be a good opportunity to refresh the icon

Light theme:

![screen shot 2014-07-24 at 10 08 07](https://cloud.githubusercontent.com/assets/1048705/3685396/6285930e-130a-11e4-9cd4-8ca8345b2992.png)

Dark theme:

![screen shot 2014-07-24 at 10 07 53](https://cloud.githubusercontent.com/assets/1048705/3685397/62b63bb2-130a-11e4-9aa2-0c4517643cdf.png)

Link to the sketch file used to design it: http://cl.ly/2W2Y0r430v0Q