Allow composer to handle dists for Git repositories

This patch adds the support of dist archives to standard git repositories (via ```GitDriver```).

This is usefull for people using satis and their own infrastructure to host repositories and wanting to publish dists archives.

This PR introduces a new attributes for VCS repositories :
```json
{
    "name": "Satis Repository",
    "homepage": "http://satis.local",
    "repositories": [
        {
            "type": "vcs",
            "url": "git@server.git:my-project.git",
            "dist": "http://satis.local/dists/my-project-%s.zip"
        }
    ]
}
```

Composer will automatically expand the given URL and append the branch/tag name to the file name.