CMake improved compiling

I've updated the CMakeLists.txt with a handful macros and automated commands, and a bit of formatting. The directories where the library and headers would be installed have been changed to `/usr/local/lib` and `/usr/local/include` so jsoncpp files won't mess up with system's libraries.

I've also added new compilation steps in the README so the library is built in a separate folder. This will keep the main repository clean while all the CMake generated files are stored in a separate folder that can be removed at any time (since you know for sure that there would be only the CMake output and not anything important for the project).

I hope you like the changes and thanks for taking your time to make JsonCpp into GitHub. I'll be using your repository as a submodule in a couple of my projects ;-)