v4.0.0

Release v4.0.0

Fixes:
 - added missing null assigner

New:
 - for_each has it's own fe_iterator which contains detail if item is object, array or other,
   and in case of object and array, it's index/key
 - for_each iteration function
 - added example utilizing for_each, and providing a dump with it

Changes:
 - completely re-factored iterators

Iterators no longer have JSON::iterator::RESULT, iterator * and -> are
fixed to result as JSON and iterator itself has functions for index/key
and most of JSON functions are made available through iterator.

Signed-off-by: Oskari Rauta <oskari.rauta@gmail.com>