Better path and URL encoding/escaping

Better path and URL encoding/escaping

* svn2svn/svnclient.py (safe_path): Creating to abstract building
paths which are safe to pass as svn command-line args. For URL's,
use urllib.quote() to URL-encode paths. For local paths, if file/dir
name includes "@", add a trailing "@" so svn doesn't get confused
about unintended text being parsed as a peg-revision.
* svn2svn/svnclient.py: Use safe_path() for all local and URL paths.
* svn2svn/run/svn2svn.py: Use svnclient.safe_path() for all local
and URL paths.