Start on a truly WP-specific XML-RPC API

Now that WordPress has its own well-specified XML-RPC API[1], let's go
whole hog and use that.

[1] http://codex.wordpress.org/XML-RPC_WordPress_API
Use our new WP API to handle lists of categories and tags

In recent versions, the notions of categories and tags have been
melded into the idea of taxonomies.  Thus the WP API deals in
taxonomies of particular types.  Change over to using our new
functions that use this taxonomy code.
Use tag names, not their slugs.

Now it might make sense to have an alternate key that searches for a
given slug that works for both Categories and Tags---in fact, I would
probably favor this---but having Categories use names and Tags use
slugs seems like a gratuitous difference.
Ignore backup files.