kcfinder\session class added

1) I've moved session init to separate class (out of uploader.php) because I think that starting and managing of session is not in uploader's responsibility.

2) It still supports "session key" definition in the config file but now also `\Iterator` objects or clean arrays.

So we can set `_sessionVar` in the config like `KCFINDER` and `SESSION['KCFINDER']` will be used.

But we can also set `$someObject` as value of `_sessionVar`. Then this object will be used for sessions.
It's very useful for better session control for example with frameworks. In this case, KCFinder won't start session and won't access `$_SESSION` super global variable directly. It will access via given object from config.