Made usage of setSessionStorage() optional

In the old version setSessionStorage() was called in Module's bootstrap method. However this prevented usage of fallback mechanism in case Redis server is not available. I removed call to setSessionStorage() from Module.php and added a few lines to README to make it up to user of this module to decide when and where to call setSessionStorage(). It allows users to implement fallback mechanism for sessions (e.g files in my case).