org-download.el: require 'url-http.
org-download.el: `org-download--dir' is decomposed into two parts.

* org-download.el (org-download-image-dir): updated the doc, it's now buffer-local.
  (org-download-heading-lvl): new buffer-local variable.
  (org-download--dir-1): new function.
  (org-download--dir-2): new function.
  (org-download--dir): path is no longer expanded.
  (org-download--fullname): updated the doc, filename is expanded here now.
org-download.el: added `org-download-yank'

* org-download.el (org-download-image): no longer inserts image from
  last kill - use `org-download-yank' instead.