Update the About message

This PR updates the content of the About message. It now automatically fetches the SHA-1 id of the git commit on which the binary is built, as well as the build timestamp.

Also updated the contributors list, and the message now points to the website.

NOTE: I used the git subtree mecanism to integrate an external lib of CMake files. You do not need to care about this mecanism, except if you want to modify some of the CMake files. In this case, ping me.

NOTE 2: The review is quite long because of the integration of the external lib. Only review commit 00a783d.