Exporter: Add taxonomy as separate node to term

[dnaber-de]

Currently the structure of an exported term looks like this:

<wp:category>
    <wp:term_id />
    <wp:category_nicename />
    <wp:category_parent />
    <wp:cat_name />
    <wp:category_description />
</wp:category>

This forces the importer to tread terms depending on their taxonomy. It would be way easier to pass the taxonomy as a separate node to the complete term:

<wp:category>
    <wp:term_id />
    <wp:category_nicename />
    <wp:category_parent />
    <wp:cat_name />
    <wp:category_description />
    <wp:taxonomy>category</wp:taxonomy>
</wp:category>