Fixing assignement for iterator wrapper second, and adding unit test

Adding first and second properties to iteration_proxy_internal

Adding support for Meson.

Test cases succeed as expected, but the example in #367 is not fully
realized yet.

An optional parameter for dump() allows to set the character to use for
indentation (default: space). In case a JSON value is serialized to an
output stream, its fill character is used (and can be set with
std::setfill).

I forgot to consider the offset.

a simple acceptor function

The documentation mismatched the actual code.

needed for VS2015.3 with /std:c++latest

Default arguments are forbidden in lambdas.

Creates all well-formed sequences of bytes up to length 4. Furthermore,
creates ill-formed sequences by removing required trailing bytes or
changing bytes. As the tests can take a lot of time, preprocessor
symbols are introduced.

As I learned in https://github.com/melpon/wandbox/issues/209, this
library is already installed at Wandbox, so we need to adjust the
examples to use `#include "json.hpp"` insteas of `#include <json.hpp>`.

std::setw needs the iomanip header (at least with GCC).

The example with the size() operator is bad: using operator[] already
changes the size of the object. The fix makes it clearer.