Sjoerd: add separate administration of temporary files created y
URLopener.retrieve() so cleanup can properly remove them.  The old
code removed everything in tempcache which was a bad idea if the user
had passed a non-temp file into it.  (I added a line to delete the
tempcache in cleanup() -- it still seems to make sense.)

Jack: in basejoin(), interpret relative paths starting in "../".  This
is necessary if the server uses symbolic links.