Consider adding `purge` method.

Consider adding `purge` method to purge pull zones, callable like so:

``` javascript
var zoneid = 12345;
vail files = [ '/foo.css', '/bar.css' ];
maxcdn.purge(zoneid);
maxcdn.purge(zoneid, files);
maxcdn.purge(zoneid, files, function callback(err, res) { 
    // ... callback code ..
});
```
