org-element: New syntax for export snippets

* contrib/lisp/org-element.el (org-element-export-snippet-parser,
  org-element-export-snippet-interpreter,
  org-element-export-snippet-successor): Use "@@backend:contents@@"
  syntax.
Update tests and comments for new snippet syntax

* contrib/lisp/org-export.el (org-export-snippet-translation-alist):
  Fix docstring.
(org-export-as): Fix code comment.
* testing/lisp/test-org-element.el: Update tests.
* testing/lisp/test-org-export.el: Update tests.
org-element: Every keyword starting with ATTR_ is an affiliated keyword

* contrib/lisp/org-element.el (org-element-affiliated-keywords):
  Remove attributes keywords from the list since they are, de facto,
  affiliated keywords.
(org-element-multiple-keywords): Remove "attr" keywords from the list
since they always belong to that list.
(org-element--affiliated-re, org-element-collect-affiliated-keywords,
org-element-interpret--affiliated-keywords): Handle special attr
keywords correctly.
org-babel-R-command is now customizable

* lisp/ob-R.el (org-babel-R-command): From a defvar to a defcustom.
Implement a Beamer back-end for org-export

* contrib/lisp/org-e-beamer.el: New file.
org-e-latex: Clean-up beamer parts in template

* contrib/lisp/org-e-latex.el (org-e-latex-template): Clean-up beamer
  parts in template.
org-odt.el: Cleanup temporary XML buffers after export

* lisp/org-odt.el (org-odt-zip-dir)
(org-odt-cleanup-xml-buffers): New.
(org-export-as-odt-and-open, org-export-as-odt)
(org-odt-init-outfile, org-odt-save-as-outfile)
(org-export-as-odf, org-export-as-odf-and-open): Use
`org-odt-cleanup-xml-buffers'.
Merge remote-tracking branch 'origin/maint'
org-e-beamer: Fix customize group and a variable name
org-export: Silence byte-compiler
org-export: Fix byte-compilation

* contrib/lisp/org-export.el (org-export-with-current-buffer-copy):
  Fix byte-compilation.
org-export: Small clean-up

* contrib/lisp/org-export.el (org-export-get-subtree-options): Small
  clean-up.
org-export: Add tests

* testing/lisp/test-org-export.el: Add tests.
org-export: Modify exclude tags and select tags keywords

* contrib/lisp/org-export.el (org-export-options-alist):
  EXPORT_EXCLUDE_TAGS (resp. EXPORT_SELECT_TAGS) becomes
  EXCLUDE_TAGS (resp. SELECT_TAGS).

EXPORT_ prefix is required for subtree properties relative to export.
Thus, this change avoids confusion.
org-export: Add a subtree property for each export option

* contrib/lisp/org-export.el (org-export-get-subtree-options): Add
  a subtree property for each export option.
* testing/lisp/test-org-export.el: Add tests.

Properties names are obtained by adding "EXPORT_" prefix to buffer
keywords.
Add links to Org mode and GNU Emacs websites in creator strings.

* org-html.el (org-export-as-html): Add links to the Org mode
and GNU Emacs websites When :html-postamble is set to 't.

* org-export.el (org-export-creator-string): Add links to the
Org mode and GNU Emacs websites.
org.el: `org-toggle-heading' skips comments and returns a message when nothing has been done.

* org.el (org-at-comment-p): New function.
(org-toggle-heading): Use `org-at-comment-p' to skip comments.

Thanks to Charlie Millar for raising this issue.
org-mobile: Added five new targets for edit nodes

* lisp/org-mobile.el (org-mobile-edit): Added handling of
  addheading, refile, archive, archive-sibling and delete edit nodes.
(org-mobile-locate-entry): olp links containing only a file are now be
  located correctly.
(org-mobile-apply): Instead of finding the location of all target
  headings for edit nodes in a separate loop, they will be found
  immediately before applying edits.

org-mobile-apply needed to be changed, as the new edit nodes can
insert new headings or delete them, thereby changing the locations
of the target headings. Thanks to Aaron Peromsik for helping with
this patch.
org-export.el (org-export-filters-alist): Fix typo in docstring.

* org-export.el (org-export-filters-alist): Fix typo in docstring.
only try to insert extant hashes

* lisp/ob-lob.el (org-babel-lob-execute): Only try to insert extant
  hashes.