Initial commit of "Plugin Interface" & supporting directory structure.

Initial commit of "Plugin Interface" & supporting directory structure.

This commit will allow follow-up contributions between contributors to
this new feature to IoTivity.

Change-Id: I41b812c048e44dded31a7fefa75f1e3830fc8daf
Signed-off-by: Joseph Morrow <joseph.l.morrow@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2091
Reviewed-by: Erich Keane <erich.keane@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/2765
Reviewed-by: Patrick Lankswert <patrick.lankswert@intel.com>
Tested-by: Patrick Lankswert <patrick.lankswert@intel.com>