added a way to enable/disable PHP support when parsing a YAML input via Yaml::parse()

PHP support when parsing a file has been disabled by default.
If you do need PHP support when parsing a YAML file, enable it via:

    Yaml::setPhpParsing(true);

As of Symfony 2.1, PHP support is disabled by default, and support will
be removed in Symfony 2.3.
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);