Fixed compatibiltiy with Python 2

Omitting positional argument specifier in `str.format` was a feature of [Python 3.1](http://docs.python.org/dev/library/string.html#format-string-syntax).

This patch adds `0` in `{}` of `encrypt` and `decrypt` to enable compatibility with Python 2. (Tested with 2.6.6 on Debian 6)