BS3: Drop the accordion for the panel (fixes #8648)

The accordion turned out to be basically the same as the panel, only not as cool. So, I've extended the panel component to include a `.panel-group` and collapsible features.

* Deletes the accordion.less file
* Extends the panel to be collapsible with our JS plugin
* Supports panel color variations

Here's how it looks now with all panel markup:

![screen shot 2013-08-12 at 11 34 52 pm](https://f.cloud.github.com/assets/98681/952903/e4fb5b76-03e2-11e3-99ce-e369f9b07493.png)
![screen shot 2013-08-12 at 11 35 18 pm](https://f.cloud.github.com/assets/98681/952904/e4f6efc8-03e2-11e3-97af-2668501b91af.png)