wrap-json: add unpacking tool for json

The functions wrap_json_unpack, wrap_json_vunpack,
wrap_json_check, wrap_json_vcheck, wrap_json_match and
wrap_json_vmatch can be used to get items in a json
object.

These functions are likely to be compatible in the
format with the unpacking functions of the library
jansson [1].

[1] http://jansson.readthedocs.io/en/2.10/apiref.html#parsing-and-validating-values

Change-Id: I18ad702498b9a70215d88b2a3c13bd0f6194bb45
Signed-off-by: José Bollo <jose.bollo@iot.bzh>