Put git clone of docopt in src

That way new versions of docopt can be swapped in
once CMakeLists.txt is configured correctly

TODO: Fix CMakeLists.txt so out-of-source build works
Right now a manual build works in the docopt.cpp directory:

clang++ --std=c++11 ==stdlib=libc++ -I . docopt.cpp examples/naval_fate.cpp -o
naval_fate

This doesn't work here since the create_single_source_cgal_program function
is called from the CGAL_CreateSingleSourceCGALProgram.cmake module

We need to add the correct header directories and targets