Improved sessions

JsonFileSessionStorage
* Added parameter to have a custom unique id function.
* json_decode returns an associative array.
* Added null check on json_decode.
* Changed the getUnusedId method.

SessionManager
* Added the date created to each session record.
* Stopped using the stdObject class.
* Added default value to expiresInSeconds constructer parameter.
* Fixed expires check in the load method.