Sticky nav css refactor

Hi @jlord Merry Christmas

Have been working on this a couple of weeks back, thought I better get it into a PR or it would never happen.

I'm pleased with these suggestions, but unfortunately I seem to have broken the chinese version - did you know? There isn't a header / footer partial in the chinese version is there? Are you thinking of making a build process for the chinese version as we have for the english version at some point?

Anyway, not sure if I or you might get around to fixing the Chinese version soon, but here's the lowdown on this PR:

Hover is nicer on the link to home:
### from this
![oldhome-link](https://cloud.githubusercontent.com/assets/2903904/5565017/5704a36e-8ed7-11e4-8e7a-4e62a3f2b02b.gif)

### to this
![newhomelink](https://cloud.githubusercontent.com/assets/2903904/5565018/5b89fb28-8ed7-11e4-8094-1b26ebf1002a.gif)

And the nav is now sticky (well it's stuck fast really):
### new sticky (stuck) nav
![stucknav](https://cloud.githubusercontent.com/assets/2903904/5565021/d6e3851e-8ed7-11e4-88e6-e2093986b02c.gif)

Also refactored the CSS quite a bit more. Fairly sure this is what has broken the Chinese pages, as I have refactored some of the HTML to match my changes to the CSS, but haven't updated the Chinese pages to match I don't think (I guess I naively assumed they would copy themselves from the english ones during the build process!)

Thanks - let me know your thoughts :-D