ME: added 'submission_count_for_today' property in xform model
ME: added 'submission_count_for_today' field in xform_serializer.py
ME: added test case 'test_submission_count_for_today_in_form_list'
Merge pull request #507 from onaio/315-precalculate-submission-num-for-current-day

315 precalculate submission num for current day