Improved clarity of the app guide

Added Editor, Terminal, and Browser icons for improved clarity.

Showed the icons at the first page of the app guide:

![icons](https://f.cloud.github.com/assets/501156/120425/9860f26a-6cea-11e2-91ee-7ce75bf41d0d.png)

Shows small icons for each of the code and terminal steps:

![icons-small](https://f.cloud.github.com/assets/501156/120426/9a6e8126-6cea-11e2-9e19-8bc1b1e12cae.png)

Had this idea to increase visual hierarchy by making the step numbers stand out:

![standout-step-numbers](https://f.cloud.github.com/assets/501156/120427/17bd87b2-6ceb-11e2-80cb-36cf5d2e6b04.png)

I only did that the App guide.