import wordpress' posts inclusive folder structure in the url

Given a WordPress XML export with the following links:

    <link>http://www.die-welt.net/2013/03/qifi-the-pure-js-wifi-qr-code-generator/</link>
    <link>http://www.die-welt.net/2013/03/opera-standards-and-why-i-should-have-stayed-in-my-cave/</link>
    <link>http://www.die-welt.net/2013/05/wheezy-ejabberd-pidgin-and-srv-records/</link>
    <link>http://www.die-welt.net/2013/05/powerdyn-a-dynamic-dns-service-for-powerdns-users/</link>
    <link>http://www.die-welt.net/2013/06/running-debian-without-unity-on-a-machine-that-is-64-bit-capable/</link>
    <link>http://www.die-welt.net/2013/07/say-hello-to-mister-hubert/</link>

And running `nikola import_wordpress --no-downloads -o foo wordpress.xml`, I get the following structure:

    % tree foo/posts/2013
    foo/posts/2013
    ├── 03
    │   ├── opera-standards-and-why-i-should-have-stayed-in-my-cave.meta
    │   ├── opera-standards-and-why-i-should-have-stayed-in-my-cave.wp
    │   ├── qifi-the-pure-js-wifi-qr-code-generator.meta
    │   └── qifi-the-pure-js-wifi-qr-code-generator.wp
    ├── 05
    │   ├── powerdyn-a-dynamic-dns-service-for-powerdns-users.meta
    │   ├── powerdyn-a-dynamic-dns-service-for-powerdns-users.wp
    │   ├── wheezy-ejabberd-pidgin-and-srv-records.meta
    │   └── wheezy-ejabberd-pidgin-and-srv-records.wp
    ├── 06
    │   ├── running-debian-without-unity-on-a-machine-that-is-64-bit-capable.meta
    │   └── running-debian-without-unity-on-a-machine-that-is-64-bit-capable.wp
    └── 07
        ├── say-hello-to-mister-hubert.meta
        └── say-hello-to-mister-hubert.wp

Previously, I would have got `20130Xsomethingsomething` as the slug, which is ugly :)