Disable un-necessary builds
sanity.json: Add another mapped JSON object for testing
sanity.json: Add additional JSON types inside mapped obj for testing
Value: Re-introduce static member null
JsonSerializer: Additional refactoring -- reading deep hierarchies

* Introduce JsonSerializer::read_value method; we now support reading
unlimited depth of JSON objects within an array top-level node. See
last object in examples/Resources/json/sanity.json. This functionality
is only present within unserialization -- support for serialization is
coming up next!

* Temporarily diable unserializer test three.
ptree/Value: Fix debug output of null value types