Base64 Decode Improvements

The existing support for decoding Freenet's encoding method based on base64 did not restore padding characters, which Freenet may omit. See [Base64,java](https://github.com/freenet/fred-official/blob/build01444/src/freenet/support/Base64.java#L15).

I also noticed that the Python [base64 functions](http://docs.python.org/2/library/base64.html) (as of Python 2.4) can take an `altchars` argument which gives alternatives for `+` and `/`.