Added option to add child categories to URL

# Summary
Child categories can now be nested in the URL. For example, assuming the category "Child" is a child of "Parent", normally the URL structure would look like this:

    Parent: /categories/parent
    Child: /categories/child

This option provides the following URL structure:

    Parent: /categories/parent
    Child: /categories/parent/child

To me this represents a logical structure, which I felt was sorely missing.

# Enable
    $Configuration['Vanilla']['Categories']['NestUrls'] = TRUE;

If it's not enabled, nothing should be affected.

# Limitations
Categories can still not have a default name, even if nested within another. E.g. "/Foo/Bar" and  "/Foo" would not work.

# Screenshots
![screen shot 2013-05-18 at 12 51 31 am](https://f.cloud.github.com/assets/22677/520732/66138c2a-bf45-11e2-84d0-a833fa95a72b.png)

![screen shot 2013-05-18 at 12 52 42 am](https://f.cloud.github.com/assets/22677/520735/88886168-bf45-11e2-94be-48ab5a60b30b.png)

P.S. Apologies for my other, similar pull request was was based the wrong branch. Would you mind deleting it?