Racket: show images with print, write and display

Up to now, we were only displaying images when printed as values by the
REPL, but not when image values were explicitly print-ed, write-d or
display-ed.  This patch solves that problem by installing (semi)
appropriate port-{print,write,display}-handler.  This is still and
incomplete solution in that those handlers (as well as the already
installed current-print-handler) don't recurse over a value's structure
and won't produce images embedded in other data structures, as discussed
in issue #49.