Simple multistage support

A quick attempt at adding multistage support (which works)

# deploy.php
It is important to define your stages after you have defined your servers, the stage is a list of servernames which are retrieved from the defined servers.
```php
// stage(name, array(servername1, servername2, ...), array('key'=>'value'), default)
stage('develop', array('development'), array('branch'=>'develop'), true);
stage('production', array('production'))->options(array('branch'=>'master'));
stage('test', array('test'))->set('branch', 'develop');
```

# commandline
```php
dep my:task [stage] [--server=server] [--dryrun]
```