Add Droidcon UK notification background

Replace the default background with a more DroidconUK-related one
Before:
![device-2014-10-23-093800](https://cloud.githubusercontent.com/assets/3942812/4758168/0318ade6-5add-11e4-805e-43cb5755b9f5.png)


After:
![device-2014-10-23-184406](https://cloud.githubusercontent.com/assets/3942812/4758159/f62437e0-5adc-11e4-9f99-49aec3218888.png)
![device-2014-10-23-184425](https://cloud.githubusercontent.com/assets/3942812/4758165/fee8ce68-5adc-11e4-8abf-3731b41e70ad.png)