XML namespace operators

[rvanheest]

To support namespaces in XML parsing/serializing, the following things were added to the library:

• node, emptyNode, stringNode and branchNode got overloads with the NamespaceBinding as the second argument. When using these overloads, the node's name is given the prefix of the NamespaceBinding.
• namespaceAttribute is renamed to attribute and no longer has the implicit NamespaceBinding; instead it is now an overload with a second parameter containing the namespace. When using this overload, the attribute's name is given the prefix of the NamespaceBinding.
• A new operator withNamespace was added to XmlSerializer and XmlPickle that adds the actual namespace(s) to a node.
  • Note that this operator was not added to XmlParser. So far I haven't gotten it to work in all circumstances. The only thing it does, though, is verify that this namespace is actually present in the xml structure. This is not a critical feature to have, so for now I left it out.

Furthermore, tests were added for these new overloads and operators. Also an example was added to demonstrate the use of namespaces in spickle.