added Cache-based session class

I wrote a Session class for Cache. Until now there was "native", "cookie" and "database" drivers, but now I wrote a "cache" driver. For example, many people may find useful to use Memcache (or any other cache) for sessions - with this it can be done.

Examples:
Session::instance('cache')->get('something')
Session::instance('cache')->set('something', 'to store')

Configuration can be done the same as with "database" driver.