improve CSS for category drop-down on desktop (bug 1083625)

# tasks completed

* simplify the CSS
* remove a bunch of unnecessary positioning changes upon hover
* remove the gap between the edges and add a `border-radius` to each nav item on the edge of the drop-down menu

# before

![screenshot 2014-10-15 21 15 43](https://cloud.githubusercontent.com/assets/203725/4657734/fa7a66e0-54ef-11e4-913d-83aaadd47bd8.png)

# after

![screenshot 2014-10-16 12 57 32](https://cloud.githubusercontent.com/assets/203725/4669502/b0b1c1c8-556e-11e4-97cc-df1e6c5c7502.png)

I'm not super pleased about how the lines lead up to the white triangle at the top, but I think overall it looks better.