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, ...), default)
stage('develop', array('development'), true);
stage('production', array('production'));
stage('test', array('test'));
```

# commandline
```php
dep my:task [stage] [--server=server] [--dryrun]
```

# todo
- [x] basic support for multiple stages
- [x] definable default stage
- [x] make stages optional
- [x] throw exception when defining stage with an undefined server
- [x] throw exception when defining stage before any server is defined