BS3: Refactor alerts to require a modifier class

![screen shot 2013-08-18 at 7 50 14 pm](https://f.cloud.github.com/assets/98681/983804/6d461d2c-087a-11e3-8e28-4f2786d5a96b.png)

![screen shot 2013-08-18 at 7 53 38 pm](https://f.cloud.github.com/assets/98681/983809/95167112-087a-11e3-891f-ed79ea1e76f3.png)

This refactors alerts to have a base class, but no default class, thus requiring a modifier class at all times. It simplifies styles for all variations since we no longer have to override the old default yellow alert styles. Each alert type has it's own CSS, thus limiting the our own overrides, and any overrides devs/designers will add in their own theming.

Docs have been updated accordingly