Fix visited link styles on bootstrap components

* Navigation pills, list groups and drop downs shouldn’t go purple when
they are visited
* Add examples to the style guide to ensure that the visited and
non-visited states remain consistent

![screen shot 2014-05-30 at 16 26 14](https://cloud.githubusercontent.com/assets/319055/3138813/ecb58faa-e8af-11e3-909e-6bfa8a921f93.png)
![screen shot 2014-05-30 at 16 26 21](https://cloud.githubusercontent.com/assets/319055/3138814/ef3b6808-e8af-11e3-8636-6b0267f61c7d.png)