rosie.svn_post_commit: more notification

Configurable notification on trunk commits.
Module re-written to improve flow of logic. It now attempts to build 2 data structures about the changeset before taking any action.
We probably should. (However, the `rose.config.ConfigNode.__eq__` method will not be able to do the double `None` logic of `config_node_1 is config_node_2`.)