[mod] do not split walk function

This implied to change the walker's API by changing the special case
on_leaf to before_leaf and after_leaf.

This wouldn't be a big deal if there was no leakage of "stop" handling
everywhere. Furthermore you must never forget to call your parent
"before" and "after" functions.

It's not pretty but the only way for now since the "visitor"'s and the
"property"'s code are mixed.
[mod] add a render walker stop test
[doc] update documentation
Merge pull request #43 from Psycojoker/walker

Update walker API to be used by redbaron