Make an option to allow gulp-ssh-deploy to not add tasks

[jwir3]

It isn't always convenient for gulp-ssh-deploy to add tasks to gulp automatically. We should also have the ability to manually run the tasks in question. This would allow us to do something like:

var gSD = new GulpSSHDeploy(
  {
    "host": "your-server.somewhere.com",
    "port": 22,
    "package_json_file_path": "package.json",
    "source_files": ".",
    "remote_directory": "/path/to/remote/deploy/dir",
    "username": "someuser",
    "ssh_key_file": "/path/to/your/local/ssh/key",
    "releases_to_keep": 3,
    "group": "remote-group",
    "permissions": "ugo+rX",
    "package_task": "someTask",
    "deploy_task_name": "deploy",
    "add_tasks": false
  }, gulp);

gulp.task('performDeployment', function() {
  gSD.runDeploymentTask();
});