Updating CMake files

[florianthonig]

What is the purpose of your code change?

I updated the CMake files to allow this library being used as git submodule.

For overview:
- I wanted to make a simple client for testing purpose with this library as a git submodule
- This library was set up as top level structure not allowing a higher level "CMakeLists.txt"
- Removed git submodule "libcurl", because it is already shipped with "cpr" git submodule
- Fixed some includes for "json" because the interface includes of the library were different
- I also noticed, that some people may build the "json" library with single header, so I changed this..
- See the changes in the README.md to see how to use this library as submodule

How did you test your change?

- I successfully compiled, linked and run my code
- I installed the library

Pull Request Author Checklist

Check all relevant items.

• [ ] My code follows the style guidelines for this project
• [ ] I have performed a self-review of my own code
• [x] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes