Throw an exception if a class name has a leading \

When a class name is prefixed with `\` PHP-DI doesn't show any warning:

``` php
return array(
    '\Foo\Bar' => DI\object(),
);
```

This will not work as FQN must not contain a leading `\`. PHP-DI should throw an exception.

Related to #242, might need a "validation pass"? Enabled only for dev environment?
