BS3: Add optional theme (fixes #8706)

## Preview

![theme template for bootstrap](https://f.cloud.github.com/assets/98681/981942/91778c10-07aa-11e3-9234-113e06e0bb1a.png)

## What's changed

Just finished the first pass at our optional theme to make Bootstrap 3 look a bit more like Bootstrap 2. It includes a first pass at adding visual effects for nearly all our components.

Specifically:

* Added a new `theme.less` file to house all our theme-specific styles (which also imports vars and mixins forr use)
* Compiles the theme file into two CSS files, `bootstrap-theme.css` and `bootstrap-theme.min.css`
* Adds new example, "Bootstrap theme", to the Getting Started examples section in the docs

I'm sure I missed something or need to tweak something else, but we can address that in time. Also keep in mind that not everything needs to be themed, so don't expect them all to appear here.

In the future, we'll do our best to make it so folks can simply change a variable and enable the theme rather than including a secondary file. For now though, this will suffice for v3.