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