Add parseIterator method to Json::Reader

Because this:

    reader.parseIterator(vec.begin(), vec.end(), root);

is way more intuitive than:

    reader.parse(&(*vec.begin()), &(*vec.end()), root);
Also, I'd rather have template code in a separate header when possible, to reduce compilation-time for those who do not need templates. But that's just a preference. Maybe compilers are getting faster?