support svn repository

I'm not sure if I can follow #518 (https://github.com/ros/catkin/issues/513) discussion precisely, but hits what I wrote for using catkin_prepare_release for svn repository. 
c91b875 update "bump version number code" section for new commit_files() and tag_repository() files and 8a1fb7 uses tag_name ( = tag_prefix + new_version ) for commit log, instead of using just a new_version