wallets: better icon for multibit

Related: #498, #493

This is a higher res image for the multibit wallet. The source is from the `.icns` file from their official github repository.

How it fits inside the square/circle. (top: after, bottom: before)
![screen shot 2014-08-04 at 13 13 18](https://cloud.githubusercontent.com/assets/189580/3796385/59855e8a-1bc8-11e4-8ba6-893d7dc265d7.png)