Helper class for handling xml:base attributes.
Usage:
- set location of the containing document by calling {@link #setDocumentLocation(String)}. This is usually done when getting setDocumentLocator SAX event.
- forward each startElement and endElement event to this object.
- to resolve a relative URL against the current base, call {@link #makeAbsolute(String)}.
External entities are not yet taken into account when determing the current base.