Added an option to close a certain session and fixed a bug

[Zlynt]

-> When receiving an OPEN request, if the flag comes null then the server crashes. Now, if the flag comes null it handles it like it was a write flag. -> Flags like 'rx' and 'r+' are handled has 'r' -> Flags like 'wx' and 'w+' are handled has 'w' -> Added a method that could close a certain session -> Improved documentation -> Improved example server code