Fix Category URL

I found an issue on the category URL.

For example, I set a category like `Node.js` to an article and deploy in a sub directory like `/subdir`.

Octopress generates the following URL:

    /subdir/categories/node-dot-js/

But this plugin generates:

    /subdir/subdircategories/node-js



This PR fixes this issue.
I referred to `Jekyll::Filters#category_link` in `plugins/category_generator.rb`.


P.S.
This logic is same as `Jekyll::Filters#category_link`, so it should be called.
(I couldn't do it, because it seems to need some big change.)