scripts: Add '-g <git remote>' to addsite command

addsite now can automatically create a git repository and push it to the
given remote.

To always use a git repository for any new site, add the following
repository template to your $HOME/.zotonic-defaults file:

    export GITREMOTE="git@bitbucket.org:username/%%SITE%%.git"
core: Remove debug message
core: Eliminate compilation warning in m_media