factor out schema builder

:Release Notes:
Prepare code infrastructure to provide new API

:Detailed Notes:
- factored out builder
- TODO: 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: I39e30dc9b39ae06d1dd53287a247f871eded7234
Reviewed-on: https://g2g.palm.com/5078
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>