Subversion support

Subversion support is added. I've tested both *git* and *svn* support in both Python 2.7 and 3.3 to make sure that I didn't accidentally interfere with existing functionality.

I had to add VCS-specific settings in order to get a tag URL. I mention it in the README:

    If you're using Subversion, than you'll need to declare the URL for your 
    repository's tag path:

      versioneer.vcs_settings['svn'] = { 'tag_url': 'https://svn_host/svn/repo/tags' }

I haven't added keyword support, yet. We can discuss this a little bit when you've had time to respond to this and the other stuff.

This PR also includes a template directory and some comments to help with understanding the flow.