Adds namespaces support for Yaf`s skeleton class generator

In case the ```yaf.use_namespace``` is enabled, the Yaf classes generator will create a valid skeleton with namespaces.

Underlined class names:
```$ php -d yaf.use_namespace=0 tools/yaf.php > /tmp/Yaf_underlined.php```

Class names with backslashes:
```$ php -d yaf.use_namespace=1 tools/yaf.php > /tmp/Yaf_namespaced.php```


P.S. Since the Yaf classes signature can be in 2 variants, the file ```tools\yaf_classes.php``` should be regenerated for both cases. Or as an option, to remove this file from repository at all.