Release Name: Change enumeration; add coordinates

Notes: This release changes the way the breath and depth first enumeration work. The methods each_depth_first and each_breadth_first now yield a node and its descendants in the specified order instead of returning enumerator objects. The previous behavior can be obtained by calling the depth_first_enumerator and breadth_first_enumerator methods. This makes the Node interface more in keeping with Ruby conventions for each functions, and eliminates unnecessary private classes in this module. This release adds dereferencing of child nodes by coordinate with the [] operator.