[PATCH]: added Converter, Encoder, Decoder, and Base64Converter

[Waldemar Kornewald]

Hi,
after having looked at Squeak again it seems they are not deriving
base64 from stream, but using a MIMEConverter framework. So, I did this
for Slate, too, but with a generic Converter, Encoder, Decoder.
Maybe one can build a media framework on top of this (*very*very*very*
simple system)? Making it MIME-specific did not make sense to me, at least.

Bye,
Waldemar