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.
the problem was when it was  single element itw as working. but when there are more checkboxes.
In line 
on_change(val,this.parentElement.innerText);
when it was setting there was no way to identify which label got selected as in no way from ui can find mapping between key and display label.
so am passing the parent of the selected checkbox using (parentElement.innerText) and 
in 
controls_[name] = option.create_control(function(value,name) 
I added name attribute so that on click 
it sets the key value for  selected checkbox.
Without this it was always setting value against the last added checkbox.
I know this is a kind of hack.
One way is I can assign id to input checkbox and name to label with the key(ex) so that I can pass it on change event.(Thats why I had added id and name in code already)
Let me know if this sound good then I can go ahead and change it.