Adding beforeSuite and afterSuite.

Hey!

I'm adding ```beforeSuite``` and ```afterSuite```, these functions can help in cases that a setup is needed to run the batches, i.e. to create a server before the batches and to close it after the batches.

```
var server;
vows.describe('HTTP').addBatch({
	'hello world server': {
		topic: function(){
			request.get('http://localhost:8080', this.callback)
		},
		'should say hello world': function(err, res, body){
			assert.equal(body, 'Hello World');
		}
	}
}).beforeSuite(function(){ 
	server = require('http').createServer(function(req, res) {
		res.writeHead(200, {'Content-Type': 'text/plain'});
		res.end('Hello World');
	}).listen(8080, this.done)
}).afterSuite(function(){
	server.close();
	this.done();
}).export(module);
```

Both ```beforeSuite``` and ```afterSuite``` must call ```this.done()``` after their setup to proceed with the batches execution.