Iterators are now implemented via const_iterators and reverse_iterator
and const_reverse_iterator now share a template class. Thanks a lot!

- added functions is_structured() and is_primitive()
- updated documentation
- updated test cases

- implemented `get_ptr` function to return pointer to value member
- overworked `get` function to support pointer types
- added test cases
- added documentation (see
http://nlohmann.github.io/json/classnlohmann_1_1basic__json.html) with
examples

In this commit, also the semantics for values skipped via the parser
callback has changed. Now, the top-level value is returned as “null”
instead of “discarded”.