lib: Add minimal JSON library

This is a minimal JSON parser (and string encoder helper).  This has
been originally written for the NATS client sample project, but since
it's a generic bit of code, it's also being provided as a library
outside the NATS application source.

It's limited (no support for arrays, nested objects, only integer
numbers, etc), but it is sufficient for the NATS protocol to work.

Jira: ZEP-1012

Change-Id: Ibfe64aa1884e8763576ec5862f77e81b4fd54b69
Signed-off-by: Leandro Pereira <leandro.pereira@intel.com>