lisp/omm.el:  Section 'Mode Definitions' moved behind 'Variables'

* lisp/omm.el: Definition of org-minor-mode moved behind definition of
  mode variables (like in org.el).
lisp/omm.el: Change three constant's default values to ""

* lisp/omm.el (org-EOL, org-BOL, org-STAR): Change default value to
  the empty string "".