Consider different API for subscriptions in LS2

:Release Notes:
C++ Wrapper extension for subscription functions

:Detailed Notes:
Add LS::JSONPayload lightweight wrapper over libpbnjson
Add LS::SubscriptionPoint class wraps subscribers list

:Testing Performed:
Unit and integration tests

:QA Notes:

:Issues Addressed:
[BHV-3671] Consider different API for subscriptions in LS2

Open-webOS-DCO-1.0-Signed-off-by: Oleksii Rudenko <oleksii.rudenko@lge.com>

Change-Id: Ie7ae18d7d90db42050d054cb32ae3d59a4b03cbf
Reviewed-on: https://g2g.palm.com/5438
Reviewed-by: DCO Verification
Reviewed-by: Sergiy Kryvonos <sergiy.kryvonos@lge.com>
Tested-by: Sergiy Kryvonos <sergiy.kryvonos@lge.com>
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>