org-odt.el: Make LibreOffice the default converter

org-odt.el: Make LibreOffice the default converter

* lisp/org-lparse.el (org-lparse-do-convert): Replace
`call-process' with `shell-command-to-string'.

* lisp/org-odt.el (org-export-odt-convert-processes): Add a new
converter which depends solely on LibreOffice and nothing
else.
(org-export-odt-convert-process): Make the above native
converter the default.

Thanks to Giles for introducing the "--convert-to" option of
soffice.exe.  The option seems to be a fairly recent
development.

http://imperfectsoftware.blogspot.in/2012/01/one-of-my-less-enjoyable-tasks-is-to.html

With this change, one can export to pdf via odt by a simple
  (setq org-export-odt-preferred-output-format "pdf")
There is no need to install additional converters.