XML backend deprecated

We're deprecating the xml backend because PostgreSQL rocks and the psql backend already implements the complete feature set of the xml backend.

I still believe in what we were searching for when we began the xml backend, however:

* a VPS server is as cheap as a cPanel shared web hosting
* PostgreSQL has a smaller footprint and better performance than mySQL (mySQL is what we were steering away from)
* writting psql queries is easier than writting SimpleXML + XPath
* the performance with multi-document queries is ought to be faster than opening and parsing a bunch of files every time.

So, for the sake of faster, cleaner code, that's what we're doing. Some cPanel web hosting services do offer PSQL. We will post a migration script before breaking compatibility -- we have several xml wikis after all. We suggest that you use the new backend in any new site you create.

If anyone is still interested in the XML backend, for whatever reason, we might reevaluate the situation. But so far everybody agrees that we should move on.

In other news, the §include block can now include only a single section of a document.

And a new §meta block has been implemented in the psql backend. It queries the whole wiki. Currently it can include or exclude documents based on their tags, we'll add more filters if/when needed.