transform yajl callbacks to schema builder methods

:Release Notes:
Prepare code infrastructure to provide new API

:Detailed Notes:
- move yajl callbacks for schema parsing to schema_build.c as visitor
  methods
- drop mentioning of yajl
- TODO: expose schema builder and parsing callbacks in public API

:Testing Performed:
unit-tests

:QA Notes:

:Issues Addressed:
[BHV-3085] Provide pbnjson API for schema construction from jvalue

Open-webOS-DCO-1.0-Signed-off-by: Nikolay Orliuk <nikolay.orliuk@lge.com>

Change-Id: I131e61a0848e62dc744abef3e280909e2798c58f
Reviewed-on: https://g2g.palm.com/5088
Reviewed-by: DCO Verification
Reviewed-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Tested-by: Nikolay Orliuk <nikolay.orliuk@lge.com>
Reviewed-by: Anatolii Sakhnik <anatolii.sakhnik@lge.com>