Added a CMake file to easily build citybuilder and link it against SFML.

With the added `CMakeLists.txt` it should be rather easy to build *citybuilder* and link it against the SFML libraries.
Here's basically what you need to do:

* [Optional] Set `CMAKE_BUILD_TYPE` to either `Release` or `Debug` depending which type you want to build for.
* [Optional] Check `SFML_STATIC_LIBS` if you want to link SFML statically.
* [Optional] Check `CITYBUILDER_STATIC_STD_LIBS` if you want to link the runtime libraries static. **Note:** SFML needs to be linked with the same option.
* [Optional] Set `CMAKE_INSTALL_PREFIX` to the directory you want *citybuilder* to get installed to.
* Set `CMAKE_MODULE_PATH` to SFML's directory `SFML/cmake/Modules` or install the `FindSFML.cmake` in your shared CMake directory.
* Set `SFML_ROOT` to the directory SFML can be found in.
* Generate a make or project file and use that to build *citybuilder*.