fix issues with org-ehtml-server

  This ensures that the original value of this variable is a string
  literal (which is assumed by the customization interface).  It also
  set's the variable as "risky", which should inhibit it's setting from
  a file local variable.  Finally this switches the :initialize function
  to a :set function which will be run when the variable is set through
  the customization interface.

  fix #16.