DESIGN: add section on how to handle Xml.Attr <> GXml.Attr
Attr.vala: parent_node, previous_sibling, and next_sibling are supposed to be NULL for Attr according to W3C
BackedNode.vala: add internal function set_xmlnode, mostly so we can replace the xmlNode a GXmlNode points to in the case of GXmlAttrs when we're manipualting them through our GXmlNamedAttrMap (which is how Elements handle them, through that)
Document.vala, gxml/Makefile.am: clean up some whitespace and remove some comments
NamedNodeMap.vala: change GXmlNamedAttrMap so that we can preserve and return old Attrs as we should according to W3C DOM
test/SerializableTest.vala,SerializationTest.vala: update test expectations because GXmlSerialization now returns Documents, not just Nodes (since our change in the memory model)