Add 'pattern' keyword to JSON Schema

:Release Notes:
Implemented 'pattern' schema keyword for string 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: Ibd78d14f3824c6fe17d40b85ca758b6dad7c2785
Reviewed-on: https://g2g.palm.com/4627
Reviewed-by: DCO Verification
Reviewed-by: Mykola Kryvonos <mykola.kryvonos@lge.com>
Tested-by: Mykola Kryvonos <mykola.kryvonos@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>