Add keyword "finally" for exceptions

This was added in PHP 5.5, see http://www.php.net/manual/en/migration55.new-features.php#migration55.new-features.finally
Add keyword for "yield"

See http://www.php.net/manual/en/language.generators.overview.php for details
Add highlighting for "trait"

This adds support for traits, which are available since PHP 5.4

Fixes #41.