Apply changes to slideshow exporter to use new `org-html-divs'

Apply changes to slideshow exporter to use new `org-html-divs'
specification.

* contrib/lisp/ox-deck.el:
  - Rename header/footer options and methods to preamble/postamble to
  be more consistent with underlining html exporter. Also remove
  included wrapper section to allow for using the element specified in
  `org-html-divs'.
  - add `org-deck-preamble-postamble-styles' variable
  - Rename `org-deck-title-page-styles' to `org-deck-styles'.
  - Rename `org-deck-title-page-* to `org-deck-title-slide-*'

(org-deck-template):
        - use :html-doctype instead of forcing html5.
        - use xhtml valid meta charset tag
        - Use elements specified in `org-html-divs'.
        - Cleanup embeded css (move into a single style section)

* contrib/lisp/ox-s5.el:
  - Use `org-html-div' container class for content
  - Change header/footer templates to remove wrapper from
  customization string.
(org-s5-toc): Use :html-container instead of hard-coded div.
(org-s5-template): use :html-doctype, :html-container and
`org-html-divs' container class.