org.el: Use `temporary-file-directory' for storing downloaded remote images for `org-display-inline-images'

lisp/org.el (org-display-inline-images): `org-babel-temporary-directory' is not used because Babel is meant for managing code blocks.  And `substitude-in-file-name' is used for fixing the double slash problem in front of a remote path.

TINYCHANGE
Merge branch 'kit-changes' of github.com:kitchoi/org-mode into kit-changes