Add 'multipleOf' keyword to JSON Schema

:Release Notes:
Implemented 'multipleOf' schema keyword for numbers validation

:Detailed Notes:

:Testing Performed:
Unit tests with memcheck

:QA Notes:

:Issues Addressed:
[ANG-341] pbnjson: Implement missing schema keywords

Open-webOS-DCO-1.0-Signed-off-by: Mykola Kryvonos <mykola.kryvonos@lge.com>

Change-Id: I0894339cff060d034c2fb4cd6ce1905c9a41b2dd
Reviewed-on: https://g2g.palm.com/4626
Reviewed-by: DCO Verification
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>
Tested-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>