Added test for namespaced inheritance.

This currently fails. The diff:

``` diff
001+ Fatal error: Delcaration of Collections\Vector::values declares return type Collection, which could not be found in /home/levijm/Projects/php-src-return-type/Zend/tests/return_hint/015.php on line 9
001- object(\Collections\Vector)#%d (%d) {
002- }
```

To be honest, it may not pass once the issue is fixed; I think the leading `\` might goof up.