added a way to enable/disable object support when parsing/dumping

By default, object support is disabled, and instead of throwing an
exception when an object is handled, null is returned.

If you do need object support, enable it via:

    Yaml::dump($data, false, true);

If you want an exception to be thrown in case an invalid type is handled
(a PHP resource or a PHP object), pass true as the second argument:

    Yaml::dump($data, true, true);

The same can be done when parsing:

    Yaml::parse($data, 2, false, true);