rez-release: RezReleaseMode.add is_valid_root() classmethod, which returns True if the release mode works with the given root path.  Previously this was handled by raising RezReleaseUnsupportedMode in __init__, but this could mask errors that should be seen by the user.  The current behavior identifies a directory as being supported by a given mode (e.g. svn mode is available if the directory has a .svn subdir) and then raises RezReleaseUnsupportedMode if there is an unexpected problem during release (e.g. pysvn is not available).
rez-release: use pysvn to get the svn changelog
rez-release: add RezReleaseMode.get_url() for retrieving the url string identifying the source of the VCS repository.  this should be used to replace rez-VCS-print-url.