Fix redirect loop.

Of course we can not just redirect to the root on denied access, because we might come from there.
Better language handling.

Instead of magically setting the language based on some setting in the database, we now use the much better approach of putting the locale in the URL. The user can switch between the available languages via links in the header. This also means that guests can switch languages for the login form now. So this commit addresses issue #3.

All URLs will change because of that change. However, the app redirects to the user's set language when no language is given by the URL, thus keeping old-style URLs valid.

This is a major change, so give it a first version bump.
Fix link behind the logo to go to the currently selected language.
Added missing translation.
Merge branch 'master' into bootstrap

Conflicts:
	app/views/layouts/application.html.erb
Use navbar as header and made greeter prettier.