Implement additional integration options

Generate the config files of the package in CMAKE_CURRENT_BINARY_DIR
instead of CMAKE_BINARY_DIR because the later will refer to the main
project's binary directory if the library is included in the project
using the add_subdirectory command.

- 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

Define the library as an interface imported library so other targets
can use the library as a dependency and use the interface properties
of the library.

fixes #233

make serialization locale-independent (fixes #228)