Show missing skill prerequisites in loadout view

Looks something like this:

![screenshot 2013-12-25 22 50 47](https://dl.dropbox.com/s/5dzp1ypy32plyn6/Screenshot%202013-12-25%2022.50.47.png)

I didn't make the prerequisite section collapsed by default because it looked icky, and also because using the space that far down the page seemed okay.

Feel free to go nuts on the code review, style nitpicks, visual design complaints, code design complaints, whatever. As mentioned on IRC, my PHP is about six years rusty :)