This Fixes #900(Comment Notification)

Fixes #900(Comment Notification)
1>Users can subscribe to notifications in the settings panel. Added another option.
2>Subscribed users will receive notification when anyone adds / modifes comment to his notebook.
3>User can anytime unsubscribe inorder not to receive any mail.
4>The mail will contain From email address which is the github registerd email of the commentor . The receiver can directly reply to that mail saying "Thanks/ or anything"
5>The recipient's email id is teh same as registered in github.
6>The subject will tell you if its a modified or new post also it will tell you who has sent and on what notebook. 
7>Mail server is configurable through conf file.
EX:
ew Post Subject :sujitbehera27 posted a new comment on your notebook [dataframe 4]
modified Post Subject :sujitbehera27 modified an old comment on your notebook [dataframe 4]

@gordonwoodhull :This is a first cut code. There are changes in many files. 
I have a feeling it needs optimization. I will sit on a second round on this.
But if you like you can review and give your comments.
why change the key name for an existing option?  isn't this key name invalid?