New launcher icon

This PR tackles issue #23 by providing a new launcher icon, which has a more distinctive silhouette and adheres closely to the android launcher icons design guidelines (the Holo ones, as there is no Material launcher icons guideline yet).

### Old launcher icon
![ic_launcher](https://cloud.githubusercontent.com/assets/153802/4710495/80a6b5d0-58aa-11e4-9f7e-0745173e4a42.png)

### New launcher icon
![ic_launcher](https://cloud.githubusercontent.com/assets/153802/4710485/7172d8c8-58aa-11e4-90df-ae0e3a156b32.png)