Shell: Replace BASE64Decoder with DatatypeConverter

BASE64Decoder is an internal prorpietary API. Java's core XML library includes
a similar function (i.e. javax.xml.bind.DatatypeConvert.parseBase64Binary()).

Fixes #16.

Signed-off-by: Or Cohen <orc@fewbytes.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>