Add getters

[Kroc]

At the moment DOMTemplate can only set data, not read it easily. In NoNonsense Forum simplexml is used to modify existing threads. It would be nicer if we could use a DOMTemplate object to read, modify and write the threads this would be more elegant.

This requires a number of additions for various functionality:

• Getting a node value, accounting for nodes that are attributes
• Initiating xpath queries starting from any current node, including returning an array of nodes (as with looping at the moment)
• Simple DOM traversal, especially with arrays, e.g. parent->child[$x]