Implement identicon representation for devices.

The first fifteen characters of device IDs are now used to procedurally
generate psuedo-unique avatars for their respective devices. The avatars
are represented using SVG elements that replace the icons previously
shown next to device names in the GUI.

(fixes #735)

![screen shot 2014-10-16 at 12 35 08 pm](https://cloud.githubusercontent.com/assets/240083/4669184/a3de97f8-556b-11e4-9dc1-6ad06af0390f.png)