documentation word choice changes
update tests to let node find _tree's source
grunt jasmine_node tweaks, maybe unnecessary
v0.2.6: implement callbacks, `_tree.on` and `_tree.off`

An `afterUpdate` event is now available, making tree finalization
plugabble, as well as proving a persistent hook to listen for all tree
updates. Callback functions can be registered either on tree creation
or via `Tree.prototype.on`. Callbacks can be unregistered via
`Tree.prototype.off`.

Each callback function receives the newly finalized tree, and executes
before the tree original operation returns. If your callback functions
directly mutate the tree, this could lead to stack exceptions.
update README and coverage for v0.2.6
add ToC to README