tests no longer rely on strict mode behaviors; update README for IE9

_tree was developed in a strict environment. restrictions are relaxed
for older browsers to let _tree function there. If used as intended,
will still provide (but not enforce) an immutable tree structure.

Test failures due to unsupported or wonkily-supported strict mode have
been modified to bypass strict mode issues entirely. I care more about
whether the tree can be modified than whether an error is thrown when
you try.

Now all tests pass in IE9.