Add initial CMake build support.

This CMakeLists.txt provides targets for unittests, perftests and all
example programs.

Note that this file does not copy the `bin` folder contents to the
target directory, which only applies if you do out-of-source builds. In
which case you may copy the folder manually, or ideally update
CMakeLists.txt to perform this. I didn't manage to get this to work.

It's possible the minimum CMake version required is actually lower than
that published, but I cannot test. Users of this file on earlier
versions should modify this if earlier versions do in fact work.

To use:

    rapidjson$ mkdir out
    rapidjson$ cd out
    rapidjson/out$ cmake ..
    rapidjson/out$ make unittests

This CMake file has also been tested with beta builds of the new CLion
IDE and works well.

You may also configure the build using a tool such as `ccmake`.

    rapidjson/out$ ccmake .