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

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();
});
```