make all addons the same height by adding a transparent background

**Before:**
![skaermbillede 2014-10-21 kl 16 36 08](https://cloud.githubusercontent.com/assets/227529/4726434/81f44fb6-5962-11e4-9c9c-7456386afd8c.png)

**After:**
![skaermbillede 2014-10-21 kl 16 40 23](https://cloud.githubusercontent.com/assets/227529/4726435/81fa076c-5962-11e4-99b1-3a94e3713112.png)