Fix chaijs/chai#141 - make menu links display block

@logicalparadox - making the guide links display block means that the majority of the hover state is clickable area. 

### Before
The blue area is the <a> tag and therefore the clickable part
![before](https://cloud.githubusercontent.com/assets/118266/4770956/fca72566-5b89-11e4-86cd-617231f9ea9c.PNG)

### After
The blue area is much more clickable now.
![after](https://cloud.githubusercontent.com/assets/118266/4770957/fcacbc1a-5b89-11e4-9a8e-31386015b42b.PNG)


Tested in Firefox 33, Chrome 40