* Add function documentation that I should have written before.
* Create separate RSSParser class to clean up RSS.php
* Create separate RSSHooks class to handle parser initialization and
public interface for parser to use.
* Move User Agent definition to $wgRSSUserAgent from a define.
* Eliminate references to $wgRSSUseGzip (since were using MW's
internal HTTP client, it didn't do anything anyway, ATM).
Re Tim's emailed review (discovered yesterday):
* Switch to DOMXPath::query() from DOMXPath::evaluate() at Tim's
suggestion.
* Move highlighting callback to its own class, RSSHighlighter, so that
it works as a callback without create_function() and other mess.
