WIP: Cmake

This should partially address #55 (closed) I have introduced a CMakeLists.txt file for the C++ API

I have added a .gitlab-ci.yml file to have CI pipelines.

I have also added a Doxygen config file with the possibility to create the documentation. It should automagically produced by the CI pipeline on the master branch.

I had to change the include of the protobuf header file the following files:

cameo-api-cpp/src/cameo/EventStreamSocket.cpp
cameo-api-cpp/src/cameo/OutputStreamSocket.cpp
cameo-api-cpp/src/cameo/Serializer.cpp
cameo-api-cpp/src/cameo/impl/ServicesImpl.h

as follows:

-#include "../proto/Messages.pb.h"
+#include "Messages.pb.h"

I think that it is much cleaner if those files are in the build/ directory. By the way, I have not found ways of getting them in the proto/ directory as it was with libtool.

If you accept the change, maybe we can also change that behaviour with libtool.