Support for SkipStepException and handling class inheritance

(This PR is based on #174.)

This PR is to add the support to allow the handler to tell the graph navigator to skip itself and proceed with the normal procedure. For instance,

``` php
use PhpOption\None;

class MyHandler implements SubscribingHandlerInterface
{
    public function serializeProxy(JsonSerializationVisitor $visitor, $entity, array $type, Context $context)
    {
        if ($visitor->getRoot() === null) {
            throw new SkipStepException('Skip This');
        }

        return array(/* ... */);
    }
}
```

Also, this PR *supports the class inheritance* such that we can specify a parent class in `getSubscribingMethods`. For instance,

``` php
class MyHandler implements SubscribingHandlerInterface
{
    public static function getSubscribingMethods()
    {
        return array(
            array(
                'direction' => GraphNavigator::DIRECTION_SERIALIZATION,
                'format' => 'json',
                'type' => 'EntityParentClassOrInterface',
                'method' => 'serializeProxy',
            ),
        );
    }

    public function serializeProxy(JsonSerializationVisitor $visitor, $entity, array $type, Context $context)
    {
        /* ... */
    }
}
```