Docs: create new callout classes

I'll open this pull request to start the discussion about new `.bs-callout-*` classes in the docs.

Starting questions:
* What kind of categories should the new callouts represent?
* Are the colors OK?
* Is on top of Getting started a good place to introduce these? Do they need to be explained?

![calloutfam](https://f.cloud.github.com/assets/3535675/705791/362cebee-dddd-11e2-936a-7ddde48f7d59.png)

Looking forward to your input!
/cc @mdo @cvrebert