Introduce callout box shortcodes for use in handbook and Code Ref articles.

Registers four new shortcodes, `[info]`, `[tip]`, `[alert]`, and `[warning]` and adds basic styling.

All four shortcodes use the basic syntax of `[open]Message[/close]`.

TODO: Add styling for the icons and any other tweaks the shortcodes' output markup.

Output for the four shortcodes currently looks like this:

![screen shot 2014-09-22 at 5 35 50 pm](https://cloud.githubusercontent.com/assets/995399/4365655/3d1369f8-42b1-11e4-9360-36abaf0b6551.png)