Add `repo` command for automatic updates on Linux

The command generates a repository for your app in target/repo. For example:

(venv) michael:~/Temp$ fbs runvm ubuntu
(venv) ubuntu:MyApp$ fbs clean
(venv) ubuntu:MyApp$ fbs freeze
(venv) ubuntu:MyApp$ fbs installer
(venv) ubuntu:MyApp$ fbs repo
Done. You can test the repository with the following commands:
    echo "deb [arch=amd64] file:///root/MyApp/target/repo stable main" | sudo tee /etc/apt/sources.list.d/myapp.list
    sudo apt-key add /root/MyApp/src/sign/linux/public-key.gpg
    sudo apt-get update
    sudo apt-get install myapp
To revert these changes:
    sudo dpkg --purge myapp
    sudo apt-key del 14094467C98844FA757D5BF1E954B8C89764F1AC
    sudo rm /etc/apt/sources.list.d/myapp.list
    sudo apt-get update

The next step is to upload the repository to a web server. Then, your users can simply execute the above commands apt-key add etc. to install your app. What's more, when you publish a new version of your app to the server, your users will automatically receive it.