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);
Merge branch '2.0' into 2.1

* 2.0:
  [Yaml] fixed default value
  Added Yaml\Dumper::setIndentation() method to allow a custom indentation level of nested nodes.
  added a way to enable/disable object support when parsing/dumping
  added a way to enable/disable PHP support when parsing a YAML input via Yaml::parse()
  [Console] fixed input bug when the value of an option is empty (closes #6649, closes #6689)

Conflicts:
	src/Symfony/Component/Yaml/Dumper.php
	src/Symfony/Component/Yaml/Parser.php
	src/Symfony/Component/Yaml/Tests/DumperTest.php
	src/Symfony/Component/Yaml/Tests/ParserTest.php
	src/Symfony/Component/Yaml/Yaml.php