org-download.el: customize `org-download-backend' to either 'wget or 'curl.

* org-download.el (org-download-image-dir): changed from defvar to defcustom
  (org-download--backend-cmd): new variable. It's auto-set.
  (org-download-backend): currently 'wget and 'curl are supported.
  (org-download-get-heading): new function.
  (org-download--dir): new function.
  (org-download--fullname): new function.
  (org-download--image): the second argument is now a filename.
org-download.el: added some docstrings.