Use definitely-unique target/project name

There exist lots of json libraries, and project/target names must
be globally unique. If someone integrated with this library in a
particularly stupid way, using a generic name like "json" might
cause a problem.

CMakeLists.txt

 cmake_minimum_required(VERSION 3.0)
 
 # define the project
-project(json VERSION 2.0.0 LANGUAGES CXX)
+project(nlohmann_json VERSION 2.0.0 LANGUAGES CXX)
 
 option(BuildTests "Build the unit tests" ON)