add yaml/yml support for config files

[spacedevin]

• if a file has an extension of .yml, check for yaml parser
• if parser, parse
• if no parser, throw exception
• provide support for both http://php.net/manual/en/book.yaml.php and http://symfony.com/doc/current/components/yaml/introduction.html

[spacedevin]

Triaged for Tipsy 2.0 (#57)

App::config() loads .yaml / .yml via ext-yaml or symfony/yaml. Closing as fixed in 2.0.