Set page dir so link tag works with pagination pages

[viv-li]

Hi there, I've not opened a new issue for this because it's essentially the same as the closed issue #104 - let me know if you'd like me to create a new issue for this PR.

To recap #104, currently the link tag doesn't work for paginated pages that aren't in the root directory. This is because when the paginated page is copied, page.dir isn't set. Then page.relative_path (which is what the link tag looks for) is in turn set by joining dir and name.

In this PR, I'm setting @dir in the set_url method after @url is set because I'm using the inherited dir method from the Jekyll Page class which uses the url to get dir.

I'm aware that this change means the set_url method doesn't just set the url. There are a few options:

1. rename the method to something like set_url_dir
2. create a new PaginationPage method called set_dir to be called after set_url is called in paginationModel.rb, or
3. leave it as is if you think this is reasonable.