ox-latex: Ignore empty example-block and src-block elements

* lisp/ox-latex.el (org-latex-example-block, org-latex-src-block):
  Ignore element if it's empty.  This fixes error "apply: Wrong number
  of arguments: max, 0".
ox-beamer: Document columns
`org-reload' also reloads export back-ends in use

* lisp/org.el (org-reload): Also reload export back-ends in use.
Remove unneeded `:initialize' function for `org-export-backends'

* lisp/org.el (org-export-backends): Remove `:initialize' function.
org-agenda.el (org-agenda-get-timestamps): Use nil as `ts-date' for diary sexpressions

* org-agenda.el (org-agenda-get-todos)
(org-agenda-get-timestamps): Use nil as `ts-date' for diary
sexpressions.
org.el (org-paste-subtree): Fix typo in docstring

* org.el (org-paste-subtree): Fix typo in docstring.
org-agenda.el (org-agenda-write): Allow writing to an .org file.

* org-agenda.el (org-agenda-write): Allow writing to an .org
file.
org-agenda.el (org-agenda-write): Don't copy headlines' subtrees when writing to an .org file

* org-agenda.el (org-agenda-write): Don't copy headlines'
subtrees when writing to an .org file.

* org.el (org-copy-subtree): New parameter `nosubtrees'.
Koma Export: rename org-export-latex to org-latex for package lists

* ox-koma-letter.el (org-koma-letter-template): rename
org-export-latex to org-latex for package lists.

These variable names were changed with the merge of the new exporter.
New commands `org-next-block' and `org-previous-block'

* ob-core.el (org-babel-next-src-block)
(org-babel-previous-src-block): Rewrite using
`org-next-block'.

* org.el (org-next-block, org-previous-block): New navigation
commands.
(org-mode-map): Bind the new commands to C-c C-F and C-c C-B
respectively.

Thanks to Bill White for suggesting this.
org.el (org-read-date-minibuffer-local-map): Use "." to go to today's date

* org.el (org-read-date-minibuffer-local-map): Use "." to go
to today's date.

Unless I'm crazy, there is a regression here, but I'm unable to find
when the regression started for now.
org.el:  Use "!" instead of "?" to see today's diary as "?" when reading a date

* org.el (org-read-date-minibuffer-local-map): Use "!" instead
of "?" to see today's diary as "?" is already bounded by
Calendar.
Merge branch 'master' of orgmode.org:org-mode
Silent byte-compiler.
org.el (org-speed-commands-default): Bind `B' and `F' to `org-previous-block' and `org-next-block'

* org.el (org-speed-commands-default): Bind `B' and `F' to
`org-previous-block' and `org-next-block'.

Thanks to Sébastien Vauban for suggesting this.
ox-html.el (org-html-format-latex): Fix conversion in non-file buffers

* ox-html.el (org-html-format-latex): Fix conversion in
non-file buffers.

Thanks to Vincent Beffara for spotting this and proposing a patch.
ox-latex: Fix docstring

* lisp/ox-latex.el (org-latex-format-headline-function): Fix missing
  parens in the docstring.
org-latex/org-html: Small changes to menu entries
ox-beamer: Small change to menu entry
ox-beamer: Add BEAMER_HEADER keyword

* lisp/ox-beamer.el (org-beamer-template): Add BEAMER_HEADER keywords
  below LATEX_HEADER.