Modified send_messages_at_time method in Chatroom class

[ghostrider22]

Solution to #29 - Added dictionary argument to send_messages_at_time().

In my previous pull request, by mistake I had added extra space before a if statement in session class, which was showing error, I have fixed that also in this pull request.

[NavpreetDevpuri]

There should be two different functions

send_message_at_time() # same massage to multiple persons
send_messages_at_time() # different message for each person

[ghostrider22]

Done sir. I have again added send_message_at_time() to Chatroom class.