Create an install target for the project

- define the project's version in the cmake listfile
- create a config and config version file for the cmake package
- install the json.hpp file and the package's targets, config and
config version files

cmake/config.cmake.in

@@ -0,0 +1,6 @@
+@PACKAGE_INIT@
+
+cmake_policy(PUSH)
+cmake_policy(SET CMP0024 OLD)
+include(${CMAKE_CURRENT_LIST_DIR}/@JSON_TARGETS_FILENAME@)
+cmake_policy(POP)