Issue #13930: Adds ability for 2to3 to write its output to a different

Issue #13930: Adds ability for 2to3 to write its output to a different
directory tree instead of overwriting the input files.  Adds three command
line options: -o/--output-dir, -W/--write-unchanged-files and --add-suffix.

Feature backports into stable release branches for 2to3 are allowed by
a special exemption:
 http://mail.python.org/pipermail/python-dev/2011-December/115089.html