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:
new 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.
The server already knows the current username, gistname, notebook title, and notebook owner, and the comment is redundant with the already existing argument to `post_comment()`.

Maybe better to just generate the email on the server?