Put back Bootstrap 2 nav list

* Pattern is heavily used in Publisher, Panopticon and other tools
* Include note in style guide about avoiding using it
Ignore DS_Store files
Include nav-list header styles

* Update to use Bootstrap 3 variables
* Remove Bootstrap 2 text shadow flourishes
* Nest styles within nav-list to avoid clashes with newer patterns
Add a real world nav-list example

* Include example from publisher, with badges
Create normal and visited link variables

* Clarify purpose of href values of links in style guide
* Make it easier to test visited links
Include visited link examples in nav-list
Merge pull request #22 from alphagov/nav-list

Put back Bootstrap 2 nav-list styles