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);
Added Yaml\Dumper::setIndentation() method to allow a custom indentation level of nested nodes.

YAML does not specify an absolute indentation level, but a consistent indentation of nested nodes only: http://www.yaml.org/spec/1.2/spec.html#space/indentation/

Projects that are generally using 2 spaces for indentation should be able to retain consistency with their coding standards by supplying a custom value for the new $indent parameter added to Yaml::dump(), or the new Dumper::setIndentation() method.

The new parameter is a backwards-compatible API addition and defaults to the previous default of 4 (which was changed from 2 via PR #2242 only recently).

Conflicts:
	src/Symfony/Component/Yaml/Dumper.php
	src/Symfony/Component/Yaml/Yaml.php
[Yaml] fixed default value
updated CHANGELOG for 2.0.22
update CONTRIBUTORS for 2.0.22
updated VERSION for 2.0.22
[Yaml] fixed typo
[DependencyInjection] fixed a bug in the YAML dumper where references where not converted to the @ notation
[Yaml] fixed unneeded BC break