add Composer support

This will allow installing Kostache as a Composer package.
Please use the Kohana module config (config/modules.php) like this:

	return array(
		'acl'                => MODPATH.'acl',
		'auth'               => MODPATH.'auth',
		'database'           => MODPATH.'database',
		// edit the path to use the Composer vendors folder
		'kostache'           => MODPATH.'../vendors/kostache',
		'minion'             => MODPATH.'minion',
		'task-migrations'    => MODPATH.'task-migrations',
		'task-upgrade'       => MODPATH.'task-upgrade',
		'orm'                => MODPATH.'orm',
		'reskue'             => MODPATH.'reskue',
		'backup'             => MODPATH.'pagoda-backup',
	);

It should still work as a submodule, but if Composer is used the
Mustache.php dependency is autoloaded by Composer instead of
recursively checked out as a submodule of Kostache.