org-odt.el: Improve locating of default styles file.

org-odt.el: Improve locating of default styles file.

* lisp/org-odt.el (org-odt-lib-dir): New constant. Directory that
provides org-odt.
(org-odt-data-dir): New constant. Directory that contains
styles and schema validation files.
(org-odt-file-extensions): All known OpenDocument extensions.
(org-odt-copy-styles-file, org-odt-begin-document-content):
Use org-odt-data-dir to locate defualt styles file.

On load a) enable rng validation of OpenDocument xml files, b)
instruct Org to open all OpenDocument files using the
platform-dependent app c) instruct Emacs to open all
OpenDocument files in archive-mode.