Updated the signature of get_body_part.

  We need to do it this way to cope with size issues with
 the returned body-part body.  Specifically calling:

        std::string( buf, len )

  Causes issues.  Ditto with utf8string.  (Presumably vbecause
 of mismatches betweeen char-count + utf-count).

  Updates #114.
  Update the use of CMessage::get_body_part()

  This is required as the signature has changed.