Implemented OCRepresentation::setChildren

Implemented OCRepresentation::setChildren

As reported in IOT 410, setChildren had a header entry, and
no implementation. This patch implements setChildren

Change-Id: I0f881dab27c09d72fd068c0b2d446bbfcc77a42b
Signed-off-by: Erich Keane <erich.keane@intel.com>
Reviewed-on: https://gerrit.iotivity.org/gerrit/561
Reviewed-by: Tim Kourt <tim.a.kourt@intel.com>
Reviewed-by: Joseph Morrow <joseph.l.morrow@intel.com>
Tested-by: jenkins-iotivity <jenkins-iotivity@opendaylight.org>
Reviewed-by: Sudarshan Prasad <sudarshan.prasad@intel.com>