This Fixes #856

This is done.
You can see an drop down of View Type with three options(view.html,notebook.R,mini.html). On selecting one of them and clicking on the share link you will be led to three urls dynamically 
Ex:
http://127.0.0.1:8080/view.html?notebook=98009b9b3b4084eddf62
http://127.0.0.1:8080/notebook.R/98009b9b3b4084eddf62
http://127.0.0.1:8080/mini.html?notebook=98009b9b3b4084eddf62

Only thing I couldn;t understand in the xisting code was 
var link = window.location.protocol + '//' + window.location.host + '/view.R/' + shell.notebook.model.user() + '/' + notebook.description;
Why were we sending it to view.R
I used the logic which was commented
// var link = window.location.protocol + '//' + window.location.host + '/view.html?notebook=' + shell.gistname();

With this all the three are working properly in my local.
@GanapatiBhat & @sneha-bharti :Can you guys verify with the shiny implementation.
@gordonwoodhull Please review and let me know if I deviated from the expected implementation.