Fix the instructions icon

Recent changes to the toolbar icon broke the picture in the instructions icon, as described in #115. This pull request fixes that. Now it looks like this: 

![115](http://f.cl.ly/items/0A3N0t3l433m0l1A2i1D/Screen%20Shot%202013-05-21%20at%2010.33.13%20AM.png "115")

@ryanstewart Are you OK with: 

1. use of the blue, active icon; and
2. the fact that the T is inverted? 

These both seem fine to me. But, if you don't think so, we'll need to get @larz0 to make us an icon without transparency.