v0.2.8: mixins

_tree now supports mixins, both on creation and dynamically. Extending
_tree just got a lot easier.

Mixins are rebound on all tree create/update operations, so be careful
with references to any _tree data structures, since they are recreated
often.
v0.2.9: better mixins

Both tree and node mixins are now possible, using the same mixin
object. The mixin must be of the format: `{tree: [...], node:
[...]}`. All nodes share the same mixing object, so be careful with
state.