[wp-xmlrpc] New method, wp.deleteCategory

[Joe Cheng]

> This is what happens when I try to end my emails with a smart
> alec line :-)

Yeah, you were kind of asking for it ;)

What happens if the promotion of an "orphaned" category results
in two categories having the same name and the same parent?
Might it make more sense to either 1) refuse to delete a
category until its child categories are all gone, or 2) delete
all the child categories automatically as well? Both of those
behaviors would be more analogous to how deleting filesystem
directories work (not that categories are that much like
directories, I suppose).