feature / 629 breadcrumbs

Proposal to fix issue #629 

It adds a breadcrumb, based on the one from "all pages", to every wiki document. I'm not too fan of the css styling I've made, but I'm unsure how we'd like the breadcrumbs to look like. Suggestions are welcome!

I'm not a ruby developer, so you may want to double check the implementation. Also, I could not find how to run the tests. Apparently my rake version is out of date, but it's the system rake, so I'm afraid of breaking stuff if I change it.