* org-attach.el (org-attach-use-annex): New function to check if git
annex should be used.
(org-attach-annex-get-maybe): New function to get a file from git
annex if necessary.
(org-attach-annex-auto-get): New defcustom to determine behavior
of org-attach-annex-get-maybe.
(org-attach-open): Automatically get attached files from git annex when
opening if necessary.
* testing/lisp/test-org-annex.el: New file for testing org-attach. Only
contains code for testing org-attach with git annex at the moment.
* mk/targets.mk: Fix cleantest target so it can delete git annex repos.
