json-for-modern-cpp: new package

A json library that aims to make json feel like a first class data
type using C++11, json for modern c++ is a easy to use, flexible,
modern json library.

Design goals include:

- Intuitive syntax. In languages such as Python, JSON feels like a
  first class data type. All the operator magic of modern C++ is used
  to achieve the same feeling in c++.

- Trivial integration. The whole code consists of a single header file
  json.hpp. That's it. No library, no subproject, no dependencies, no
  complex build system. The class is written in vanilla C++11.  All in
  all, everything should require no adjustment of compiler flags or
  project settings.

- Serious testing. The class is heavily unit-tested and covers 100% of
  the code, including all exceptional behavior. Furthermore, it is
  checked with Valgrind that there are no memory leaks. To maintain
  high quality, the project is following the Core Infrastructure
  Initiative (CII) best practices.

Signed-off-by: Adam Duskett <Adamduskett@outlook.com>
[Thomas:
 - use "json-for-modern-cpp" everywhere as the name of the package
 - add <pkg>_INSTALL_TARGET = NO since it's a header only library]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>