Implemented filesystem cache, one and only (see #1415)

See https://github.com/fabpot/Twig/blob/master/lib/Twig/Environment.php#L333 and https://github.com/fabpot/Twig/blob/master/lib/Twig/Loader/Filesystem.php#L166 . Timestamp makes sense since it is used to check if SOURCE file was modified AFTER cache was written. In our case I's a moment of template caching time.

Of course, it will never happen for this Cache, yet imho it's better than returning 0 or something since it gives an example of how the cache should behave.