expression_tree 2.0 released

OK, so I kept working on it... I couldn't help it. I still felt it could be better. And it now is!

1. The namespace expression_tree was introduced and the class expression_tree was renamed to tree.
2. The expression_tree class now uses policy-based design which is much clearer than what was before.
3. The node class is officially public, so you can make copies of nodes (and, thus, entire sub-trees) and assign them back to trees.
4. Don't worry, it's all in the documentation and the sample code.