Better C++ recognition

* ob-C.el: Added =C++= to =`org-babel-load-languages'=
automatically after loading =C=.
* org-src.el (org-src-lang-modes): Added association between
language =C++= and =`c++-mode'=.

=C++= is added to org-babel-load-languages to make
`org-babel-demarcate-block' recognize =C++= as a proper language.
removed modification of org-babel-load-languages

* lisp/ob-C.el: Don't modify `org-babel-load-languages' from ob-* files.
org-src-lang-modes in block demarcation options

* lisp/ob-core.el (org-babel-demarcate-block): Include
  org-src-lang-modes in block demarcation options.
more robust sbe macro
Fix generating LaTeX formula images

* lisp/ox-odt.el (org-odt--translate-latex-fragments): Fix imagemagick support.
* lisp/ox-html.el (org-html-latex-environment)
(org-html-latex-fragment): Fix imagemagick support.
* org.el (org-create-formula-image-with-imagemagick): Generate correct
size formula image.
(org-format-latex-header): Change pagestyle command position.
* doc/org.texi (@LaTeX{} fragments)
(Previewing @LaTeX{} fragments)
(Math formatting in HTML export)
(Working with @LaTeX{} math snippets): Add document about creating
formula image with imagemagick.
org-pcomplete: Fix completion for block boundaries

* lisp/org-pcomplete.el (pcomplete/org-mode/file-option): Remove
  spurious colons from block boundaries.
Merge branch 'maint'
fix bug in ob-sh when dealing with list variables

* lisp/ob-sh.el (org-babel-sh-var-to-string): Fix bug in ob-sh when
  dealing with list variables.
make java executable configurable for ditaa blocks

* lisp/ob-ditaa.el (org-babel-ditaa-java-cmd): Make java executable
  configurable for ditaa blocks.