Display debug mode indicator in menu drawer

Without this there isn't an easy way to determine if the app is in debug
mode or not. This change was prompted by realizing halfway through
testing the app that it was in release mode rather than debug mode.
Fortunately I didn't cause any damage in the meantime.

[Screen shot](https://f.cloud.github.com/assets/1094445/2390249/0aa2faf8-a958-11e3-875b-2f46ded80110.png)