org-xhtml.el: Support for captioning and labelling of equation images

org-xhtml.el: Support for captioning and labelling of equation images

* contrib/lisp/org-xhtml.el
(org-export-xhtml-do-preprocess-latex-fragments): Factored out
from the earlier org-export-xhtml-preprocess.
(org-export-xhtml-preprocess-latex-fragments): Check if
backend is xhtml. If yes, delegate to
org-export-xhtml-do-preprocess-latex-fragments. This is hooked
to org-export-preprocess-after-blockquote-hook.
(org-export-xhtml-preprocess-label-references): Factored out
from the earlier org-export-xhtml-preprocess.
(org-export-xhtml-preprocess): Call
org-export-xhtml-preprocess-label-references. Note that
processing of latex fragments is no longer done here.
(org-xhtml-format-image): Honor captions, label and attributes
even for images that are created by latex fragment processing.
(org-xhtml-unload-function): Unhook from
org-export-preprocess-after-blockquote-hook.

Thanks to Myles English for registering this requirement.
http://lists.gnu.org/archive/html/emacs-orgmode/2011-08/msg01116.html