Fix subtree() cannot cooperate with rsearch()

1. subtree() hasn't taken care the original bpointer of it's root node
which broke rsearch().
2. Add a if-condition on rsearch() to make sure it will break if
tree.root (string) equals current (string)
Merge pull request #33 from mondwan/fix_subtree_and_level

Fix #32: subtree() cannot cooperate with rsearch()