Add HTML id and class attributes to MenuLinks

Hi Johan,

This is the change | mentioned in my email - add an id and class attribute to any menulink.

I've added a very simplistic spec to prove that it's working - and I've tested it in my own project which I need this functionality for.

Hope you don't mind, but I've tweaked the admin CSS, and added a couple of lines to the README as well. I've also removed some trailing whitespace, although this isn't intentional - my editor does it for me, but I thought it was nice to have tidier code. Feel free to just cherry-pick the functional commits if you're not interested in any of these extras!

If I've missed anything, or you're not happy with the changeset, just let me know.
-Chris