Broke TreeBuilder up into two separate classes.

The TreeBuilder class has been broken up into XML::TreeBuilder and
AST::Processor.

The Processor class converts AST nodes into Oga::XML instances. Unlike the
TreeBuilder class it does not build an actual tree and as such is not that
useful on its own. It is extended by the TreeBuilder class to allow the
construction of an actual DOM tree.

The upcoming pull parsing API will also use the Processor class, hence the
TreeBuilder class was split in two.
Basic Processor handler for (document) nodes.