Improve admonition icons

This changes the admonition icons to the following:

* Notes are indicated with a pencil.
* Tips are indicated with an academic cap.
* Cautions are indicated with a danger sign sign.

In addition, I replaced the green background for tips with a blue one, to prevent a new color from being introduced and so that only cautions stand out more. (I tried playing with more subtle variations, including ones with a white background, but they didn’t work out well.)

Compare old and new:

![new-admonition-icons](https://cloud.githubusercontent.com/assets/6269/2595696/9b310992-ba9d-11e3-81b3-c07da49106cf.png)