Evaluate (and implement?) CMake build system

CMake seems to be a possible option for a new build system for DOLFIN. The main advantages compared to our current Scons-based system are that we would rely on a maintained and active buildsystem (compared to Simula Scons which is no longer in active development), configuration files are shorter and easier to understand, graphical user interfaces (cmake-gui) are available for the configuration step, and built-in support for building native packages for Windows and Mac.