handler reference counting

- handler reference counting
- parser returns parsing result
Removed DefaultHandler typedef

Removed DefaultHandler typedef. JSON::Parser is now default constructed
with built-in parsing handler (i.e. without requirement that user
explicitly provides handler). Handler is now a SharedPtr so old code
with handler on the stack would cause undefined behavior, hence best to
break it now in development release ... apologies to anyone affected.
Merge remote-tracking branch 'origin/feature-json-refcount' into develop